You've got a fucked up skybox. Have you tried deleting the map and redownloading it?
Basically that's the type of stuff you see when you're outside of a maps boundaries so I would heavily suggest just completely deleting the map file and redownloading it as this just looks like a missing texture. If that doesn't work try just loading up flatgrass and see if such a bug occurs and if so you might have to reinstall gmod. If that doesn't work, try to manually download a skybox file for your map and replace it in the materials directory. If that doesn't work, I can't help you darl.