https://github.com/ReplayMod/ReplayMod#building

Building

Make sure your sub-projects are up-to-date: git submodule update --init --recursive

For compiling 1.7.10, you must run ./gradlew :jGui:1.7.10:setupDecompWorkspace > :1.7.10:setupDecompWorkspace once after the initial clone. This may take quite some time.

No IDE

You can build the mod by running ./gradlew build (or just ./gradlew bundleJar). You can then find the final jar files in versions/$MCVERSION/build/libs/.
You can also build single versions by running ./gradlew :1.8:build (or just ./gradlew :1.8:bundleJar) (builds the MC 1.8 version).

IntelliJ

Ensure you have at least IDEA 2020.1.
Build the mod via Gradle as explained above at least once (./gradlew compileJava should be sufficient). This will ensure that the sources for all MC versions are generated.
Then import the Gradle project from within IDEA: File -> Open -> build.gradle -> Open as Project
Finally configure IDEA to build everything by itself instead of delegating it to Gradle (cause that is slow): File -> Settings -> Build, Execution, Deployment -> Build Tools -> Gradle -> Build and run using: IntelliJ IDEA

Eclipse

It's gone! The steps for Eclipse are simpler - Eclipse already builds by itself. Just right-click -> Import -> Existing Gradle Project.