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.