How to fix the missing map error
1. Go to your garrysmod/garrysmod directory
2. Go to downloads/maps and find your specific map name and delete the file/filles stated in your connection error.
3. Launch GMOD and join the server
If this still doesn't work go to your GMOD settings>Other and make sure you set "When a game server tries to download custom content to your computer set it to "Allowed all custom files from server"