BMM has C3 systems. It is standard rules level stuff, so it is included. Heck, it has Boosted C3 too.
But a featured mechs having C3 would be weird for newer players, me thinks. C3 is fine for variants but not feature units, unless they are outright designed around it, like the Celestials were. The Gunslinger is very much a stand-alone unit. Should it have C3 variant? Probably, but then again so should far more units than there is actually right now. Really think this is a case of working out with your fellow players whether custom C3 modifications are allowed or not.
I don't see much point in making a yet another C3 Gunslinger variant though, as there's two C3 master variants already. A heavy sniper that can carry a C3 computer without major sacrifices? That is nice.
Sure the Clan lasers would be nice addition but ultimately the Gunslinger is probably fighting at longer range anyway, not to mention it keeps BV down considering how costly C3 is.
Kinda glad the boat-face is no longer that boat-y in the new art. Gunslinger McBoatface is no more.