I know there's a basic rule against uploading screenshots from games and all that- But say I were to have animated something *purely* in Gmod (example being this: https://mega.nz/folder/HNV1mCQQ#DxVDLUu6jm0bsT79GjJlbA/file/aIsh2Khb) and wished to upload it. Would it be taken down since it's technically not a render?

I've never really understood why a poor quality render made in SFM is fine but a high quality screenshot of gmod/second life/etc gets deleted

There are over 200+ images with the Garry's Mod tag that hasn't been taken down, so I'm assuming it's possible your work won't be taken due to being "screenshots". You can try and upload your work and see if it stays or gets taken down. You could also try and ask Mario for help.