It's because you downloaded it from dropbox or something else. Follow these instructions and do what I say.
Computer > C Drive > Program Files (x86) > Steam > Steam Apps > Common > Garry's Mod > garrysmod > maps > delete: rp_downtown_v4c_v2_drp_ext.
Now you've deleted that try and join the server again. If still not. Download rp_downtown_v4c_v2_drp_ext from the workshop.
steamcommunity.com/sharedfiles/filedetai...59&requirelogin=true
I hope it helped.
- Regards