Gunner is primarily a ranged damage class that has access to physical and elemental damage, with an extra assortment of weird, janky skills, and the most broken skill in the entire game. There's an absolute ton of weird things you can do with them, but I typically end up just building for their elemental attacks plus Action Boost, the aforementioned broken skill. In theory, you could use a Gunner as a semi-support with their bind-inflicting Snipes and Medic Bullet, but on the other hand, you could also just, I dunno, use a Hexer for that. On the other hand, basically everyone that uses a Gunner is going to be using it because that's Frederica's class in Story mode, so talking about party composition with them is a bit weird and not applicable to 99% of players.


Stats

(Full stat table)

Common Passives: ATK Up, HP Up
Gathering Skill: Take

Gunnners have pretty okay STR, above-average TEC/LUC spread and TP, average AGI, below-average HP and VIT. All of these stats are pretty in line with what you'd expect from a back row physical damage dealer, I imagine. Their STR isn't optimal if you're going for maximum damage, but on the other hand they have the highest TP of any of the physical damage classes.


Gun Mastery


Required to learn gun skills. Increases damage dealt with guns.

Head Snipe / Arm Snipe / Leg Snipe


Deals ranged STR-based stab damage to one enemy. Attempts to bind the target's head (Head Snipe), arms (Arm Snipe), or legs (Leg Snipe). Has an 80% speed modifier and a base accuracy of 133% at all levels.
These have far more utility in the Story party...but even then, only if you're more concerned with having disables instead of getting as much damage out of Frederica as possible. I guess these end up having pretty high bind chances, effectively, if you combine it with Action Boost.
These are pretty okay for basic lockdown and conditionals, and the damage isn't that terrible either for the cost.

Fire Rounds / Ice Rounds / Volt Rounds


Deals ranged STR-based stab + fire (Fire Rounds), stab + ice (Ice Rounds), or stab + volt (Volt Rounds) damage to one enemy. Has an 80% speed modifier and a base accuracy of 98% at all levels.
The basic Rounds'll serve you well enough for damage throughout, like... Pretty much most of the game until mid or late 5th Stratum, I'd say. The Charged elemental shots take far too much SP and cost too much TP for any point before that, in my opinion.
This is essentially what Gunner uses for a pretty good chunk of the game. There's not really much else that they do.

Charged Fire / Charged Ice / Charged Volt


Deals ranged STR-based stab + fire (Charged Fire), stab + ice (Charged Ice), or stab + volt (Charged Volt) damage to one enemy. Has a 0% speed modifier (will go last in the standard turn order) and a base accuracy of 98% at all levels.
These will be your main damage skills in the endgame and postgame, but you really should not level them up before that point I mentioned in the Rounds commentary. You will drain your Gunner of both their TP in battles and of skill points they need for a really good passive and a really good skill.
These cost a shitload of TP, but they're effectively just Rounds but slower and bigger. Combined with Action Boost, they're terrifyingly strong but also terrifyingly expensive.

Charged Shot


Deals ranged STR-based stab damage to one enemy. Has a 0% speed modifier (will go last in the standard turn order) and a base accuracy of 98% at all levels.
Eh. Charged Shot at level 10 deals as much as a Charged elemental shot would deal at level 10 when hitting a 125% vulnerability, but at the same time, it can't take advantage of stuff like the Fantasias. There's basically no situations I can think of where you'd want to opt for pure physical damage over elemental damage.
There's a few situations where there just isn't an elemental weakness to exploit, in which case this is the go-to attack since it edges out Charged Elements.

Penetrator


Gives a chance for the user's attacks and skills to pierce enemy rows. If the user is already targeting an enemy in the back row, the attack/skill's damage is increased by 1.5x.
You need Penetrator level 3 for a very good passive and the most broken skill in the game, but I wouldn't level it up past that.
It's okay, I guess. Pierce is probably the worst of the three area of effect variants, but Gunner's other area of effect option is pretty expensive and somewhat unreliable.

Recharge


Gives the user a chance to refund TP used for a skill.
Recharge is a fucking fantastic passive, and at rank 10, gives your Gunner 30% more TP on average. It's far better than that, though, because it's possible for Recharge to roll a lot of times, and refund a lot of TP. It's extra fun when combined with Action Boost!
This is really good for Action Boost, given how badly it drains your TP pool.

Ricochet


Deals multiple instances of ranged STR-based stab damage to random enemies. Has an 80% speed modifier and a base accuracy of 88% at all levels.
Ricochet's big problem is its lower-than-average base accuracy means it's nowhere near as plug-and-play as the elemental Rounds or the Charged elementals. If you have some way of mitigating it (accuracy buffs, evasion debuffs, panic/leg binds), it can actually be pretty decent! Without it, though, you really shouldn't bother using it.
This costs way too much for what's effectively a way worse Falcon Slash that can hit a target more than once. It's theoretically higher damage than Charged Shot on single target, but it costs more and I've never been one to trust my output to dice rolls.

Action Boost


On the next turn, the user will perform their selected action multiple times. Has no speed modifier at all levels.
Holy fuck Action Boost. This skill is ludicrously fucking broken, because it skews the action economy so heavily in your favor. At rank 10, Action Boost means that for, say, every turn an enemy gets, whoever's using Action Boost gets 1.5 turns--over the course of 2 turns, the AB character has now taken 3 turns, over the course of 4, they've taken 6, you get the idea. There's absolutely nothing preventing you from spamming this just every other turn, unlike Accelerate in EO2U, so go fucking bonkers.

Oh, yeah, also, remember the Grimoire system exists! If you're willing to put in the effort, you can put this shit on all of your damage dealers. At that point you've basically broken the game wide open because all of your damage dealers now have a permanent 1.5x damage boost.
This skill is dumb and breaks the game. I honestly don't like it because it reminds me of EO3's Charge metagame. At least it's normally locked behind Story?

High Caliber


Increases the user's critical hit damage.
why are there so many of these things
It's a crit skill. Crits are only on ATTACK commands. Gunner's probably the only one with any reason to take it, I guess.

Vulcan Stance


Increases the damage of the user's normal attacks, as well as making them target all enemies, for a set amount of turns. Has an 80% speed modifier at all levels.
There are far, far better ways of dealing with random encounters than Vulcan Stance.
This is okay if you put points into High Caliber and have a critical chance increase skill on a Grimoire Stone, but it still takes a turn of setup and you still spent SP on a crit skill.

Double Strike


Gives the user's normal attacks a chance to hit twice.
please stop with all of these stupid ways to buff up normal attacks
Hey! If you take every single possible ATTACK skill maybe it'll become okay! Hahaha! Haha! Ha...

Medic Bullet


Restores one party member's HP and cures any status ailments they might have.
Medic Bullet's problems are:
Shoot the enemies instead. Single target lock is awful and so's that TP cost.

Cover Fire


When the user is in the back row and uses the Defend command, all characters in the front row have their HP restored.
Sacrificing a damage dealer's turn for some paltry HP recovery? Ugh.
You could put this on a healer via a Grimoire Stone, I guess? You're still using DEFEND in the back row, though.

Shell Shock


Attempts to stun all enemies in the back row. If successful, Shell Shock also reduces attack, defense, evasion, and accuracy.
This is completely useless in almost every situation I can think of, but especially FOE and boss fights.
There's like a single enemy formation in the game where I would actively want this skill a lot. It's a nasty formation, but it's also the only one in the game I can think of off of the top of my head.