In open space, with no gravity and no air, you wouldn't be able to fire a gun in space. You would die first. But hypothetically, for the sake of this answer, it would blow the person firing backwards. Whatever they were trying to hit would most likely not be hit, but the bullet would float around for a while after zooming for about three seconds.
no in a vacuum u can not fire a gun because there's no Oxygen to ignight the black powder and it would go but would soon slow down if you could get it to fire because there are still particles ie: some Oxygen nitrogen and all that to slow it down that what NASA had a problem with when thay were trying to keep there space ship in low orbit.