Im not all that impressed with the grav weapons. I think they are too situational. Sure it will mess up TEq and its good againt MEq but blowes against orks,, guard, nids, demons, cultists, dark eldar, most elder... Its a cool option but for the same 15pts a plasma gun will wound most things in the game on a 2+ ap2 and it can actually glance/pen upto av13 with a +1 to the damage chart.
How many pts are the grav-cannon centurions? Cause 90 pts of cultists could effectively tarpit them the whole game.