The Hunter Class itself is one of them as it's good for beginners and you'll be very strong. But since you started the game as a Gunner, I would recommend Braver as the class has two branches that goes into two different directions of whether you want to main the bow and arrow or Katana. Especially since the Bow has two different styles depending on how you will fight with it. There's a rapid fire option that will make simply tapping the attack button and shooting fast really strong as well making rapid fire Photon Arts stronger so you could probably still stick to a more gunner-like style. However that was only my intuition on this rapid fire branch since I main the Katana in the Braver Style and I only use the Bow Charge Shot Branch (which doesn't take long to max out) because I don't normally shoot often. The Charge shot is more beginner friendly and is best at being able to let you shoot from the far rear easier, especially if you're fighting against enemies that are much stronger than you.
As for the Katana style, even though I main this fighting style of the Braver Class, it's a bit hard for me to explain, but if you really like swords and fast swings and a good way to mix up charge attacks with it, this will be the best way to help you get a bit more comfortable in fighting close quarters come back. You'll even be allowed to defend with this weapon, so you can block attacks and making you take very little damage from them or perfect guard to counter attack which will give you a huge damage boost if you managed to fill its focus gauge making you the strongest person in the field (depending on your teammates) for a short while as long as you keep hitting enemies.
Fighters are what I considered to be the the strongest class, especially if you use the fists as your main weapon. However, I think without any kind of subclass (except for maybe the magic/tech classes) you'll probably be a bit squishy (So I think your Gunner Class being a subclass might be okay for this, emphasis on "might"). But the fist weapon has this good ground pound Photon Art which you can use to stun enemies around you and cause major damage around you, especially for low hanging flying enemies that happened to be near you if you used it from the air. Anyway, Fighter Class also emphasis on dodging and attacking at the right time as well and with the fist you'll probably give out the most damage to the enemy, you might even beat it to a pulp before it has a chance to move. This class is a powerful class to try out, but it may be a bit harder than the other two classes to master, since I think it's a bit less forgiving on damage you can take. I'm not completely sure since I didn't start Fighter from scratch as it was my subclass.
Anyway, those are the classes to try out from easiest to a little hard (for you) Braver, Hunter, and Fighter.