[RussEfarmer]

Update: Tried borderless however the resolution makes the text, chat, and menus really small and that's more annoying then the tab out issue. Rip

I can't stress enough that gmod is complete garbage at making settings changes on the fly. If you applied the setting, quit the game, and started it again with it still not behaving, I'm not sure. Can't exactly say a game from '04 was designed to run at 4k..

EDIT: Did some digging, many other people have the issue with 4k. There's no solution I'm aware of other than turning the resolution down.