Step 1: Navigate to Steam/SteamApps/Common/GarrysMod/garrysmod/addons
Step 2: Select ALL files within this folder and press "DELETE"
Step 3: Navigate to Steam/SteamApps/Common/GarrysMod/garrysmod/cache/workshop
Step 4: Select ALL .gma files (leave the resources folder) and press "DELETE"
Step 5: Navigate to Steam/SteamApps/Common/GarrysMod/garrysmod/cache/lua
Step 6: Select ALL files within this folder and press "DELETE"
Step 7: Navigate to Steam/SteamApps/Common/GarrysMod/garrysmod/download
Step 8: You guessed it, select ALL files within and press "DELETE"
Step 9: One last time, navigate to Steam/SteamApps/Common/GarrysMod/garrysmod/downloadlist
Step 10: Select ALL files within and press "DELETE"
for those that want to free up space on your computer before downloading the new content pack