I was having that issue or a few days, I JUST fixed it though. Go to your games library, right click G-mod, and select "Browse local files", then find the file that says "factory reset" and double click. You should get a menu pop up asking if you want to proceed, press Y on your keyboard for yes and your game should totally reset and clear out any bugs that may be causing the Lua errors. This will not uninstall your addons by the way, so don't worry about having t redownlaod/install anything. Hope this helps!