Speaking as a gunner myself, you can't go wrong with Rath Soul. It's a fantastic set. A lot of tough enemies deal fire damage, too, so that extra fire resistance will be great.
I'm not a fan of the Artian S line. Bonus Shot is a help and a hindrance at the same time. On one hand, more dps for less ammo. On the other, every additional shot locks you in place for that much longer.If you're playing with 3 others, that may be okay because the enemy may not attack you. If not, you're stunlocking yourself into place for far too long.
On an unrelated note, I tried for 50 minutes to get that Basarios glitch for Earth Crystals on my own and still haven't done it. First tried with LBG... nope. Used one with RF Crag and Normal 2s. For one, the head is tough to hit. Two, once you do hit it the right amount, you have to trip it with Normal, then reload and hit his head again, which is very very difficult to do in time. I couldn't do it. So I switched to hammer, which I've never used before. That was much easier to figure out, but now I have no clue when he's going to get dizzy.
Ugh. I'm going to have to do it with a group.