The Leech gun (and missile) is misnamed. It doesn't drain power, but sends a power surge through the target's electronics systems. Theory would say antigrav is also zotted, but we're not given the inside view of leeched ships to know for sure whether gravity is off when ship systems are zotted offline.
As for the seatbelt thing, gravity controls aren't responsible for keeping people from getting squashed during acceleration. Protection against accelerations is provided by inertial dampeners (which also give fighters handling similar to atmospheric aircraft, baring special features like autoslide). In one of the novels (I think ER, but not sure... maybe FF) there's a comment from one of the fighter pilots that the dampener doesn't act instantaneously, so pilots will feel themselves jerked around some as they maneuver, from their ship going one way while inertia keeps the pilot's body going on its previous vector, until the dampener catches up.