How would a gun, that uses anti-gravity as propellant, work? Is it even practical?
The problem is that antigravity is ill-defined. Do you need some kind of negative-mass matter to generate it? Is it as weak as gravity? In that case, you'd need many, many earth-equivalent masses of negative matter to produce anything like a useful acceleration to fire a weapon, which would make it utterly impractical.
In a scifi context, when people say "antigravity", they really mean some kind of force like magnetism that works on any matter, with a fairly strong force being generated by a fairly small device. Calling it "antigravity" just seems to be a callback to classic scifi, or marketing, or just a lack of imagination. Given that such things are entirely handwavium, you can declare it to work however you like (but I wouldn't call it a "propellant", any more than electricty or magnetism is the "propellant" in a railgun or coilgun).
Since its non-magnetic, magnetic 'deflectors' can't bounce it off.
Firstly, railgun and coilgun projectiles don't need to be magnetic. In fact, they don't even need to be conductive... they can be pushed by a suitable and discarable "sabot" that separates from the projectile when it leaves the barrel of the weapon. Guns with chemical propellants could likewise fire any old material. Magnetic deflectors couldn't stop such projectiles, either.
Secondly, if you have a means of accelerating things via "antigravity", then what's to stop you making "antigravity deflectors" to bounce back any kind of projectile?
Thirdly, trying to use magnetism to deflect a bullet requires one hell of a magnetic field. Like, massively heavy field generators, big power supplies, big coolant reserves if you're using anything other than room-temperature superconductors. The field will also be brain and electronics scrambling, and will apply forces to any conductive or magnetic materials within its volume. Sure, you may have deflected a bullet, but you've just sucked a light-fitting out of the roof that's hit you in the head at bullet speeds, so you're not exactly winning.