Galactic Weapon doesn't change the behavior of the hero's main weapon. Like the rest of the unlockables in Overwatch 2, it's purely cosmetic, giving them a unique purple coloring.
TVS Jupiter is available in 7 colours: Lunar White Gloss, Meteor Red Gloss, Twilight Purple Gloss, Titanium Grey Matte, Starlight Blue Gloss, Galactic Copper Matte, Dawn Blue Matte.