I’m planning on uninstalling Counter Strike: Source, HL2 episode 1, and HL2 episode 2 to clear up some space on my hard drive. If I do this, will I lose the models and props from those games in Gmod?
No. As long as you have those games registered to the same steam account as the Garrys Mod you should be fine.
If you still have the gfc files, it will work.
If you keep hl2 or any other source game you’ll be fine.