This is a 100% fix for the map differs problem. IF you still have the issue after doing all of these, you 100% did not do all these correctly.
1. Make sure your game is closed.
2. Delete the following folders in Garry's Mod.
- D:\steamapps\steamapps\common\GarrysMod\garrysmod\cache
- D:\steamapps\steamapps\common\GarrysMod\garrysmod\downloads
- Find the map in D:\steamapps\steamapps\common\GarrysMod\garrysmod\maps and delete that (it might not be there but if it is delete it).
3. IF you manually subscribed to our collection through the workshop (which we do not recommend) unsubscribe from the workshop collection (WITH GMOD OPENNNNNN). After unsubscribed, reopen gmod and resubscribe to everything. Once that is done downloading restart your game and join the server.
4. Also look in the addons folder to see if you might have extracted it somewhere else manually.
5. If you get the same error after all this, you might have moved it somewhere else manually and a FULL gmod reinstall might just be quicker to fix this.