Never attack an ally.
Gunners are a relatively new addition to the series—guns first appeared in use by two party members in Final Fantasy VII, and were not made a formal Job class until Final Fantasy X-2. Gunners specialize in hitting opponents from afar, usually disabling them. Another popular Gunner ability is shooting magic out of their guns, causing elemental damage.
Also, Gunners have very high Accuracy, rarely missing their targets. The downside is that Gunners do not deal as much damage as other Jobs, since guns do not usually rely much on the Gunner's actual Attack stats. However, they deal as much damage in the back row as they do in the front. Gunners somewhat parallel the Rangers seen in earlier entries of the series.
Barret Wallace and Vincent Valentine wield guns as their weapons. Palmer also has the ability to fire bullets charged with Mako from his gun when fought, doing elemental damage. Also, Rufus Shinra wields a Shotgun with one hand on two occasions, when he was fought and his encounter with Kadaj in Advent Children.
Irvine Kinneas uses various shotguns to attack, and his Limit Break Shot makes use of different types of Ammo to deal damage to opponents in a variety of ways. Laguna Loire uses a sub-machine gun in battle.
Though their abilities are based on the Gambler, the Corsair is able to wield a variety of guns, including their own special hexaguns. The Corsair also receives access to the Quick Draw ability at level 40, which allows them to endow bullets with special effects through the use of elemental cards.
Sazh Katzroy wields two pistols in battle, which can join together to form an assault rifle. Lightning's Blazefire Saber can change into a gun, which she uses during combos. Lebreau and Gadot use machine guns for the small duration that the player controls them.
The Gunner class is exclusive to the moogle race. It uses both elemental and status-inflicting attacks. What separates the Gunner from other far-range Jobs is the fact that the Gunner's attack reach is twice as far as other classes.
The Fusilier class is the renamed version of the Gunner. Other than the name, the class has the exact same role as in Tactics Advance.