I chronically had this with island 2013 on sgm. Disconnect, go to the workshop link for dust2 ( https://steamcommunity.com/sharedfiles/filedetails/?id=2529619821 ), unsubscribe and resubscribe, let gmod re-mount it, and re-connect. Might require a restart of Gmod but not in my experience. Perhaps a permanent solution can be found, but this is the temporary fix that worked for me. To note: dropping donor BSP's directly into the gmod directory did not work for me.
Well this is closed for multiple reasons. One of them being the user is banned now, but this was also a "them problem" of them having the wrong/3rd party version of de_dust2