Not much to say other than what the title already says, I'm using the latest version. I actually cant even paste the crash report since it's so frickin long :(

Then use one of the dozen pastebin services, i.e. hastebin, paste.gg, paste.ee, gist.github.com, etc.

I also need to know if you are in a production environment or you are developing a mod.

Production environment, using it because the mod Ars Nouveau needs it.
Here's the error message (yes its pretty bloody long): https://paste.gg/p/anonymous/636e3612089443d5bfd253e1ac62fc7b

Use pastebin. Not sending us a a crash log is about as helpful as demanding a doctor help you without telling him what symptoms you have.

True, cant even paste it at pastebin tho
I could of course just paste the first 100 lines or something since that should be enough to determine the problem, so here it is:
https://pastebin.com/AyHUtFtH

There's literally no reason why you cannot upload the full log. Is this in dev or prod?

Yes there is a reason, it exceeds the max file size for a free pastebin account, version of mod is 1.16.5-3.0.10 (a release)

Make sure you are using Java 8 and redownload the jar to make sure as your jar looks broken.

Oh I'm using jdk 13, will try with jre 8

Yeah we target 8 due to the launcher shipping with 8. Anything past is Use At Your Own Risk

Alright, good to know. Putting it on the homepage on curse would probably be a good idea to avoid unneccessary crash reports from people like me :D

This as assumed thing as Forge itself has the same policy

Whoops, my bad.
I'll perform a test run and then I'll close the issue if I don't get any more issues with geckolib during startup

No issues with geckolib remaining, seems like the issue was just me not using java 8. For anyone else having the same issue, java 13 is not recommended :)