1. Delete the file : zworld-afterlife_content_109691377.gma
You find it here:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons\zworld-afterlife©_content_109691377.gma
You find it here:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons\zworld-afterlife©_content_109691377.gma