Just to let you know (even if Mekanism doesn't support 1.12 anymore), compilation fails on 1.12 branch. Dependencies (deobfCompile) cannot be found. When those dependencies are added as jar files, I get another error from forgeGradle. That's certainly not an issue on Mekanism's side but well, worth mentioning.

Error:

`> Task :compileApiJava FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Could not resolve all files for configuration ':forgeGradleMc'.

Could not find forgeSrc.jar (net.minecraftforge:forgeSrc:1.12.2-14.23.5.2847-PROJECT(Mekanism)).
Searched in the following locations:
https://files.minecraftforge.net/maven/net/minecraftforge/forgeSrc/1.12.2-14.23.5.2847-PROJECT(Mekanism)/forgeSrc-1.12.2-14.23.5.2847-PROJECT(Mekanism).jar`

Steps to reproduce:

1. git checkout 1.12
2. add jar dependencies in a libs directory.
3. replace deobjCompile instructions with compile files in build.gradle. Something like compile files('libs/CraftTweaker2-API-4.1.19.548.jar', 'libs/CraftTweaker2-MC1120-Main-1.12-4.1.19.548.jar', 'libs/gson-2.7.jar', 'libs/MCMultiPart-2.5.3.jar', 'libs/RedstoneFlux-1.12-2.1.0.6.jar', 'libs/ZenScript-4.1.19.548.jar')
4. gradlew

If you don't send the full crash log or the latest.log we can't do much

No crash nor latest.log as the problem occurs at compile time (not runtime).
Here's the full output from gradle:
`> Task :buildSrc:compileJava NO-SOURCE

Task :buildSrc:compileGroovy NO-SOURCE
Task :buildSrc:processResources NO-SOURCE
Task :buildSrc:classes UP-TO-DATE
Task :buildSrc:jar UP-TO-DATE
Task :buildSrc:assemble UP-TO-DATE
Task :buildSrc:compileTestJava NO-SOURCE
Task :buildSrc:compileTestGroovy NO-SOURCE
Task :buildSrc:processTestResources NO-SOURCE
Task :buildSrc:testClasses UP-TO-DATE
Task :buildSrc:test NO-SOURCE
Task :buildSrc:check UP-TO-DATE
Task :buildSrc:build UP-TO-DATE

Configure project :
This mapping 'stable_39' was designed for MC 1.12! Use at your own peril.
WARNING: You are using an unsupported version of ForgeGradle.
Please consider upgrading to ForgeGradle 4 and helping in the efforts to get old versions working on the modern toolchain.
See https://gist.github.com/TheCurle/fe7ad3ede188cbdd15c235cc75d52d4a for more info on contributing.
#################################################
ForgeGradle 2.3-SNAPSHOT-dfb52d0b
https://github.com/MinecraftForge/ForgeGradle
#################################################
Powered by MCP
http://modcoderpack.com
by: Searge, ProfMobius, R4wk, ZeuX
Fesh0r, IngisKahn, bspkrs, LexManos
#################################################
Found AccessTransformer: crafttweaker_at.cfg_CraftTweaker2-MC1120-Main-1.12-4.1.19.548_at.cfg
Found AccessTransformer: mekanism_at.cfg

Task :deobfCompileDummyTask
Task :deobfProvidedDummyTask
Task :sourceApiJava UP-TO-DATE
Task :compileApiJava FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Could not resolve all files for configuration ':forgeGradleMc'.

Could not find forgeSrc.jar (net.minecraftforge:forgeSrc:1.12.2-14.23.5.2847-PROJECT(Mekanism)).
Searched in the following locations:
https://files.minecraftforge.net/maven/net/minecraftforge/forgeSrc/1.12.2-14.23.5.2847-PROJECT(Mekanism)/forgeSrc-1.12.2-14.23.5.2847-PROJECT(Mekanism).jar

• Try:
  Run with --info or --debug option to get more log output. Run with --scan to get full insights.

• Exception is:
  org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration$ArtifactResolveException: Could not resolve all files for configuration ':forgeGradleMc'.
  at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.rethrowFailure(DefaultConfiguration.java:1054)
  at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.access$1700(DefaultConfiguration.java:123)
  at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$ConfigurationFileCollection.getFiles(DefaultConfiguration.java:1028)
  at org.gradle.api.internal.file.AbstractFileCollection.iterator(AbstractFileCollection.java:76)
  at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.iterator(DefaultConfiguration.java:439)
  at org.gradle.api.internal.changedetection.state.DefaultFileSystemSnapshotter$FileCollectionVisitorImpl.visitCollection(DefaultFileSystemSnapshotter.java:257)
  at org.gradle.api.internal.file.AbstractFileCollection.visitRootElements(AbstractFileCollection.java:282)
  at org.gradle.api.internal.file.CompositeFileCollection.visitRootElements(CompositeFileCollection.java:206)
  at org.gradle.api.internal.changedetection.state.DefaultFileSystemSnapshotter.snapshot(DefaultFileSystemSnapshotter.java:142)
  at org.gradle.internal.fingerprint.impl.AbstractFileCollectionFingerprinter.fingerprint(AbstractFileCollectionFingerprinter.java:54)
  at org.gradle.internal.fingerprint.impl.DefaultCompileClasspathFingerprinter.fingerprint(DefaultCompileClasspathFingerprinter.java:46)
  at org.gradle.api.internal.changedetection.state.CacheBackedTaskHistoryRepository.fingerprintTaskFiles(CacheBackedTaskHistoryRepository.java:358)
  at org.gradle.api.internal.changedetection.state.CacheBackedTaskHistoryRepository.createExecution(CacheBackedTaskHistoryRepository.java:163)
  at org.gradle.api.internal.changedetection.state.CacheBackedTaskHistoryRepository.access$100(CacheBackedTaskHistoryRepository.java:76)
  at org.gradle.api.internal.changedetection.state.CacheBackedTaskHistoryRepository$1.getCurrentExecution(CacheBackedTaskHistoryRepository.java:123)
  at org.gradle.api.internal.changedetection.changes.DefaultTaskArtifactStateRepository$TaskArtifactStateImpl.getStates(DefaultTaskArtifactStateRepository.java:200)
  at org.gradle.api.internal.changedetection.changes.DefaultTaskArtifactStateRepository$TaskArtifactStateImpl.isUpToDate(DefaultTaskArtifactStateRepository.java:94)
  at org.gradle.api.internal.tasks.execution.SkipUpToDateTaskExecuter.execute(SkipUpToDateTaskExecuter.java:50)
  at org.gradle.api.internal.tasks.execution.ResolveTaskOutputCachingStateExecuter.execute(ResolveTaskOutputCachingStateExecuter.java:54)
  at org.gradle.api.internal.tasks.execution.ValidatingTaskExecuter.execute(ValidatingTaskExecuter.java:59)
  at org.gradle.api.internal.tasks.execution.SkipEmptySourceFilesTaskExecuter.execute(SkipEmptySourceFilesTaskExecuter.java:101)
  at org.gradle.api.internal.tasks.execution.FinalizeInputFilePropertiesTaskExecuter.execute(FinalizeInputFilePropertiesTaskExecuter.java:44)
  at org.gradle.api.internal.tasks.execution.CleanupStaleOutputsExecuter.execute(CleanupStaleOutputsExecuter.java:91)
  at org.gradle.api.internal.tasks.execution.ResolveTaskArtifactStateTaskExecuter.execute(ResolveTaskArtifactStateTaskExecuter.java:62)
  at org.gradle.api.internal.tasks.execution.SkipTaskWithNoActionsExecuter.execute(SkipTaskWithNoActionsExecuter.java:59)
  at org.gradle.api.internal.tasks.execution.SkipOnlyIfTaskExecuter.execute(SkipOnlyIfTaskExecuter.java:54)
  at org.gradle.api.internal.tasks.execution.ExecuteAtMostOnceTaskExecuter.execute(ExecuteAtMostOnceTaskExecuter.java:43)
  at org.gradle.api.internal.tasks.execution.CatchExceptionTaskExecuter.execute(CatchExceptionTaskExecuter.java:34)
  at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter$1.run(EventFiringTaskExecuter.java:51)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:301)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:293)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:175)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:91)
  at org.gradle.internal.operations.DelegatingBuildOperationExecutor.run(DelegatingBuildOperationExecutor.java:31)
  at org.gradle.api.internal.tasks.execution.EventFiringTaskExecuter.execute(EventFiringTaskExecuter.java:46)
  at org.gradle.execution.taskgraph.LocalTaskInfoExecutor.execute(LocalTaskInfoExecutor.java:42)
  at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareWorkItemExecutor.execute(DefaultTaskExecutionGraph.java:277)
  at org.gradle.execution.taskgraph.DefaultTaskExecutionGraph$BuildOperationAwareWorkItemExecutor.execute(DefaultTaskExecutionGraph.java:262)
  at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$ExecutorWorker$1.execute(DefaultTaskPlanExecutor.java:135)
  at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$ExecutorWorker$1.execute(DefaultTaskPlanExecutor.java:130)
  at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$ExecutorWorker.execute(DefaultTaskPlanExecutor.java:200)
  at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$ExecutorWorker.executeWithWork(DefaultTaskPlanExecutor.java:191)
  at org.gradle.execution.taskgraph.DefaultTaskPlanExecutor$ExecutorWorker.run(DefaultTaskPlanExecutor.java:130)
  at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:63)
  at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:46)
  at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:55)
  Caused by: org.gradle.internal.resolve.ArtifactNotFoundException: Could not find forgeSrc.jar (net.minecraftforge:forgeSrc:1.12.2-14.23.5.2847-PROJECT(Mekanism)).
  Searched in the following locations:
  https://files.minecraftforge.net/maven/net/minecraftforge/forgeSrc/1.12.2-14.23.5.2847-PROJECT(Mekanism)/forgeSrc-1.12.2-14.23.5.2847-PROJECT(Mekanism).jar
  at org.gradle.internal.resolve.result.DefaultBuildableArtifactResolveResult.notFound(DefaultBuildableArtifactResolveResult.java:27)
  at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.CachingModuleComponentRepository$LocateInCacheRepositoryAccess.resolveArtifactFromCache(CachingModuleComponentRepository.java:339)
  at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.CachingModuleComponentRepository$LocateInCacheRepositoryAccess.resolveArtifact(CachingModuleComponentRepository.java:294)
  at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.ErrorHandlingModuleComponentRepository$ErrorHandlingModuleComponentRepositoryAccess.resolveArtifact(ErrorHandlingModuleComponentRepository.java:183)
  at org.gradle.api.internal.artifacts.ivyservice.ivyresolve.RepositoryChainArtifactResolver.resolveArtifact(RepositoryChainArtifactResolver.java:79)
  at org.gradle.api.internal.artifacts.ivyservice.resolveengine.artifact.DefaultArtifactSet$LazyArtifactSource.create(DefaultArtifactSet.java:178)
  at org.gradle.api.internal.artifacts.ivyservice.resolveengine.artifact.DefaultArtifactSet$LazyArtifactSource.create(DefaultArtifactSet.java:165)
  at org.gradle.api.internal.artifacts.DefaultResolvedArtifact.getFile(DefaultResolvedArtifact.java:142)
  at org.gradle.api.internal.artifacts.ivyservice.resolveengine.artifact.ArtifactBackedResolvedVariant$DownloadArtifactFile.run(ArtifactBackedResolvedVariant.java:153)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:301)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:293)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:175)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.access$900(DefaultBuildOperationExecutor.java:48)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$ParentPreservingQueueWorker.execute(DefaultBuildOperationExecutor.java:343)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable.runOperation(DefaultBuildOperationQueue.java:230)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable.access$600(DefaultBuildOperationQueue.java:172)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable$1.call(DefaultBuildOperationQueue.java:209)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable$1.call(DefaultBuildOperationQueue.java:203)
  at org.gradle.internal.work.DefaultWorkerLeaseService.withLocks(DefaultWorkerLeaseService.java:154)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable.runBatch(DefaultBuildOperationQueue.java:203)
  at org.gradle.internal.operations.DefaultBuildOperationQueue$WorkerRunnable.run(DefaultBuildOperationQueue.java:177)
  at org.gradle.internal.operations.DefaultBuildOperationQueue.waitForCompletion(DefaultBuildOperationQueue.java:116)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.executeInParallel(DefaultBuildOperationExecutor.java:138)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.runAll(DefaultBuildOperationExecutor.java:111)
  at org.gradle.internal.operations.DelegatingBuildOperationExecutor.runAll(DelegatingBuildOperationExecutor.java:41)
  at org.gradle.api.internal.artifacts.ivyservice.resolveengine.artifact.ParallelResolveArtifactSet$VisitingSet.visit(ParallelResolveArtifactSet.java:60)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration.visitArtifacts(DefaultLenientConfiguration.java:256)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration.access$500(DefaultLenientConfiguration.java:69)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration$2.run(DefaultLenientConfiguration.java:231)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:301)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:293)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:175)
  at org.gradle.internal.operations.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:91)
  at org.gradle.internal.operations.DelegatingBuildOperationExecutor.run(DelegatingBuildOperationExecutor.java:31)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration.visitArtifactsWithBuildOperation(DefaultLenientConfiguration.java:228)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration.access$200(DefaultLenientConfiguration.java:69)
  at org.gradle.api.internal.artifacts.ivyservice.DefaultLenientConfiguration$1.visitArtifacts(DefaultLenientConfiguration.java:133)
  at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$ConfigurationFileCollection.getFiles(DefaultConfiguration.java:1025)
  ... 43 more

• Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 5.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/4.10.3/userguide/command_line_interface.html#sec:command_line_warnings

BUILD FAILED in 3s
4 actionable tasks: 3 executed, 1 up-to-date
`

Forge's maven moved, old ForgeGradle doesn't know this.

You'll need to either use a patched FG version or update it to use FG3 or FG4.