how to fix this?
Gmod Content (don't worry, looks shady af but safe, used for years by many communities)
You'll need CS:S Game Content / Maps and put them in "..\steamapps\common\GarrysMod\garrysmod\addons" so you end up with:
"..\steamapps\common\GarrysMod\garrysmod\addons\css_maps_addon_dec2015\maps" and ofc game content as well.