Guns RPG crashes when used with geckolib and works when geckolib is removed.
---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 7/2/22 4:13 AM
Description: Rendering overlay

java.lang.NullPointerException: Rendering overlay
at software.bernie.geckolib3.util.json.JsonAnimationUtils.getObjectListAsArray(JsonAnimationUtils.java:198) ~[?:3.0.75] {re:classloading}
at software.bernie.geckolib3.util.json.JsonAnimationUtils.getAnimations(JsonAnimationUtils.java:47) ~[?:3.0.75] {re:classloading}
at software.bernie.geckolib3.file.AnimationFileLoader.loadAllAnimations(AnimationFileLoader.java:27) ~[?:3.0.75] {re:classloading}
at software.bernie.geckolib3.resource.GeckoLibCache.lambda$reload$0(GeckoLibCache.java:70) ~[?:3.0.75] {re:classloading}
at software.bernie.geckolib3.resource.GeckoLibCache$$Lambda$6377/869058765.apply(Unknown Source) ~[?:?] {}
at software.bernie.geckolib3.resource.GeckoLibCache.lambda$null$5(GeckoLibCache.java:88) ~[?:3.0.75] {re:classloading}
at software.bernie.geckolib3.resource.GeckoLibCache$$Lambda$6668/1740125095.get(Unknown Source) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1574) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) ~[?:1.8.0_51] {}

At first glance it appears that it's caused by geckolib directly - this mod stores it's animations in animations directory which is used by geckolib too. These incompatible files should be propably ignored by geckolib.
I'll investigate it further today once I get to pc, but this issue may be better suited for geckolib repository.

It's caused by geckolib - it expects that all files in animations directory are gecko animation files and disregards possibility of other mods using the very same directory for their animations and simply skipping invalid files. Nothing to fix on our side.