I understand that Honor Harrington is total Space Opera, but admittedly that makes me shakes my head a bit. Gravity is pretty inefficient as a propulsion system for such small devices.
I guess it could theorhetically be done, but I think by the time you could make gravity efficient enough to use in pistols or rifles, it'd be easier to just use gravity directly as a weapon. Instead of spinal mounts or lasers, you'd have "mass collapsers" or something.
There´s always the question how easy it is to project gravity, the inverse square law being what it is. Generating gravity in a missile tube to fire a missile is one thing, generating gravity strong enough to damage a target a million kilometers away - without it damaging by own ship in the process - could be an entirely different challenge.
For that matter, a "mass collapser" missile warhead would not be too feasible, either, since existing warheads are effective at ranges in the hundreds of thousands of kilometers, and point defense becomes disproportionately more effective the closer a missile approaches - inverse square law again.
The closest they get, IIRC, is that their counter-missiles - small missiles intended to intercept bigger missiles with anti-ship warheads - do not carry a warhead at all, but are intended to intersect their own gravity drive field with that of the anti-ship missile, which results in destruction of both missiles. (This mutual drive field kill only works if both fields are within a range strength range relative to each other - a missile could not destroy a starship by drive field intersection)
Or rather, that´s the closest intended use of gravity technology. At one point, the protagonists remote-control a small craft to power up its gravity drive field while it´s still in the hangar of its mothership - with predictably spectacular and catastrophic results.