The 40K universe is a seriously vast place, just what kind of a specific setting'd be fitting for a Gmod server?
Hive City RP would just be more grimdark reskin of HL2RP, and Imperial Guard RP shooting NPCs and event characters.
I played on one server set during the new lore of Indomitus Crusade, and it was... actually a good idea. The premise was a mixed commune of two ships from various Imperial organizations and societal statuses, travelling to the second half of the Milky Way rendered lost due to a galaxy-spanning Warp-rift denying them the light of the Astronomican (basically GPS required to do warp-jumps for Imperium's FTL).
It was legit interesting, picking up PDF/refugees from planets undergoing daemonic invasions, grabbing supplies and whatnot. If only some retarded ownership drama didn't split the project in two that snuffed out both of the resulting halves, it could've gotten past playtesting stage.
Still, it could be a great concept, were it not for the fact most non-humansized and -shaped models in Workshop have terrible animations due to using default Valve skeleton that, surprise-fucking-surprise, pair poorly with any models not designed for default IK rig.