This was my personal fix.

Step 1: Go to "steamapps\common\GarrysMod\garrysmod" delete "addons" and "cache" folders.

Step 2: Now go to "steamapps\workshop\content" and delete "4000" folder.

Step 3: Start garry's mod, you will notice in console your addons aren't available or missing but its ok, the game recognized that fact. Close the game.

Step 4: Go to your library, garry's mod, properties and verify integrity of game files

Step 5: Wait until the instalation finish n' you're ready to go!