Delete the bugged map from your Gmod folder and try re-downloading it when you connect to the server.
Sometimes, the map will only download partially, before something on the server causes you to lose connection mid-download.
Things like that leave you with a partially downloaded map file. When you try to connect to the server again, it scans your maps folder to see if you have the map already downloaded.
When it sees it's there, it tries to compare it to the servers version. Clearly, since it's a corroupted map file, it's going to give you the "missing map" error, since it's not the same as the server versions.
So yeah. Just delete the bugged map from your Gmod folder and you should be fine.