I’ve seen someone say something about TF2 maps crashing the game, but I’ve noticed that gm_flatgrass and some community maps don’t load.
It’s a TF2 update that broke map loading in general, just unmount TF2 in the main menu until Gmod can be fixed, doing so will allow you to load maps again, but obviously you won’t have any Team Fortress 2 content
I found the specific file that breaks it and made an addon to replace it. It’s a work-around for now, you’ll be missing some particles. You can still mount TF2 with this.