As far as I'm aware, the 'GMG' part of the small turrets is some kind of error. The turrets are actually factionalised by house. Anyone can use any, but the ones you buy on New Tokyo/Roppongi are yellow spikey things, the ones you buy on New Berlin/Bonn are green ringed things, and so on. The house difference is merely aesthetic. Stat-wise they are all the same, and yes, I believe slightly better than one of the 2.00 large turrets.