The gravity blaster would be a different sort of gun.

When the mouse is held, the player would be held in place and many small beamlike projectiles would come forwards while the gun charged. These projectiles would deal small bits of knock back, but little to no damage. They would travel very slowly, but would have huge range and knockback similar to valiance's charge at 5*.

When the gun is charged in similar time to a gran faust charge, it shoots a larger projectile that deals a status effect and passes through enemies. I'm thinking that freeze and fire would be good statuses.

Upgrading the weapon would result in better status, more range and more knock back. The base weapon would be either a boss reward, or come from catalyzer, starting at the star.

If the charge+attack combo proves too hard to code, the weapon could be given one shot that lasts like ten seconds and reloads almost instantly to make it more conventional.