This error means the map your game loaded does not match the server’s copy. It often appears after a map update or when two addons contain different versions of the same map.
If you are joining the server
- Write down the .bsp filename in the error message.
- Ask the server owner for the Workshop page of the exact map they use. Subscribe to that item and let Steam finish its download.
- Close Garry’s Mod fully, open it again and try joining.
- If it still fails, disable another Workshop map addon only if you know it contains the same map filename. Restart Garry’s Mod and test again.
Subscribing directly to the server’s map is a useful first check for an outdated Workshop download. It is not a guaranteed fix for every mismatch. Keep unrelated addons in place while you find the conflicting map.
If you own the server
- Check which Workshop item supplies the map in your collection. Remove duplicate map sources from the collection if they use the same .bsp filename.
- In Files, check garrysmod/maps for a manually uploaded copy of that map. If it is an old duplicate, download a copy, then remove only that old duplicate .bsp file.
- Restart the server and check Console for successful Workshop downloads. Ask the affected player to retry with the same Workshop item.
For map setup, see changing a GMod map. If you need help, send support the full error, map filename, Workshop item URL and whether other players can join.
Reference: a reported Workshop map update mismatch in Facepunch’s issue tracker, Facepunch’s server Workshop guide.
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