I did a test in Infected Space.
3 runs with 3 different careers & ships.
3x DHC, 1x DC, 3x Turret -> Graviton Amplifier hit 68 times
2x DBB, 1x Temporal Disruption Device, 1x Beam Array, 1 x KCB, 1x Turret -> Graviton Amplifier hit 29 times
6x Beam array, 1x Romulan Plasma Torp, 1x KCB -> Graviton Amplifier hit 56 times
The overall hit rate of the 3 ships is hard to compare. While the escort only fires cannon weapons the cruiser also had DEM + Plasma procs and various pets and abilities such as tractor beam and grav well count towards the hit rate of the science vessel. But i guess the Omega graviton Amplifier does not proc on the latter.
So the hit rate of the 'main weapons' and thus the hit rate of the Graviton Amplifier is highest with cannons and definitely lowest with DBBs on a sci ship.