The gunner dress sphere is the default dress sphere of Yuna in the game, which means you'll already have it and are free to use it throughout the entire game.
Commands
Attack
Trigger Happy
Gunplay
Item
Abilities
Trigger Happy
Description: Deals multiple hits using the R1 button. AP: 0 MP: 0 Required: none
Potshot
Description: Inflicts damage on a single enemy. AP: 20 MP: 8 Required: None
Cheap Shot
Description: Attacks, ignoring the defence of the target. AP: 30 MP: 8 Required: Potshot
Enchanted Ammo
Description: Inflicts magical damage on the enemy. AP: 30 MP: 8 Required: None