Gunner is hard in no small part because finding bullets is a nightmare. The backfire and jamming problem also fucked me more than once and if you want to have a cannon wielder you have to prep for that literally during party creation by giving them a fuckton of strength to hold the actual weapon and cannonballs, of which there are only a few.
My advice would be to play it like a backline throwing or archer character, but keep in mind that their damage is less reliable and you can't get back missed shots. I often used my normal ranged characters for long ranged harass when I suspected enemies were ahead, you do that with a gunner and you'll be out of bullets when it counts. The biggest thing is that you have to ask yourself if you need to be shooting for every encounter, bullet economy is very tight and unforgiving.
On the flip side, you won't run out of ammo like a traditional ranged character where you get like 5-10 attacks per fight without picking up ammo so you get to skip the power curve that finding another arrow/rock/bolt provides. All your damage is available right away and you can shoot as much as you like without it forcing you to path back over terrain in circles. This is huge for a few fights where you're stuck in long corridors and is very nice in large melees where you have lots of enemies and trying to pick up spent ammo might be tough. It also helps against enemies with lots of hp who just soak your shots and then make the ranged character useless.
Another nice thing that allows them to play very different is that guns don't scale with any stats. So unlike a normal character you can get away with spending very few points in your main trait. Guns are great if you want a double utility character like an alchemist-mage or just a super mage.
Also useful to keep in mind that their projectiles have no travel time. Against fast dodging enemies this is huge. Guns are super consistent dps against them as a result.