Turn off the compressor and see if the problem persists. If it goes away, it’s probably because the G is sending a hotter signal than the other strings and the compressor is gating it. Is It the same on both pickups? If it is, I’d be looking at either the nut, something loose on the bridge, if it’s not, then you have a faulty pickup. All in all, it sounds like a probable return to the shop.