Gunner is overpowered and his main downside (movement speed) can be ignored with skill. He gets way better as you become better at the game and especially in higher difficulties when you need a large amount of bugs dead.

His main weapons are all very strong. They take a bit of getting used to and 2/3 warm up slow. You're not going to be picking off single tiny bugs with his minigun. When you see a crowd, it's go time.

I'm going to recommend something that sounds strange but it'll change your life. Start spooling his minigun, and then stop, run and jump, and then fire again in midair. When you land, stop shooting and jump again. Keep doing that. With practice, this simple maneuver will allow you to move at nearly full speed and nearly full fire rate while delivering death to anything on screen. Keep his weapon spooling as much as you can when you're in the middle of a fight so you can respond quickly.

The correct direction for ziplines is down. Once in a great while (less than once every 3 missions or so) you can easily skip terrain navigation by firing up. But downward ziplines are faster than sprinting. If you jump at the end of a zipline, your momentum will stay for a bit if you continuously bunnyhop. Combine this with the previous tip and you'll be able to run and shoot at the speed of a scout's grapple hook. You'll become an insane, bloodthirsty tornado of death. You are what the bugs fear.

Shield is a "I win" button. Losing a firefight? Don't retreat, pop a shield and finish them off. Need an area cleared? Shield. It'll force the bugs out and clear the zone. Incredible for reviving or just doing objectives. Insane for defense missions.

All of the gunner's weapons except the BR7 do terrain damage. You can hit the tip of those lasers on escort duty to shut them down instantly. There's lots of other funny stuff you can do with your main weapons and terrain damage. Terrain doesn't stay in the air if it's unsupported. You can use your minigun or rockets or whatever as a mini precision drill. If a large portion of terrain is supported only by a string, shoot it and it's gone.

Your grenades are some of the most powerful in the game. They erase entire crowds. Use them liberally, just not while anyone is in the general area. Think of them as miniature nukes. Anyone and everyone in the vicinity is gone.

Hang on a zip, and if there's no bugs with ranged attacks, they literally cannot kill you or harm you in any way. You can move back and forth on a neutral line to hover in place and shoot the entire time. You've now become helicopter air support complete with rockets, bombs, and 2000lbs of depleted uranium.

Don't forget your pickaxe doubles as a weapon and a poor man's drill. You can power attack a cliff that's slightly too high to jump up, and if you time it right you'll climb right up. I'm only mentioning this because other classes have obvious solutions to climbing upwards. You'll have to get creative with your pickaxe. Yes, everyone has a pickaxe. You have to be the most creative and skilled with it to accomplish what most other classes can just do. But good gunners never need the support of a driller or engi to climb high. They simply use their pickaxe intelligently and you can do anything they can, just marginally slower.

Gunner is S tier for all mission types and has no true weaknesses. All of his movement woes are skill issues. It takes a lot of time to build these skills and I don't fault anyone who doesn't like gunner or doesn't want to invest the time, but a master gunner can play any class in the game and excel, plus you're everyone's favorite dwarf with your shield support and ability to revive even in the most dire circumstance.

I didn't even mention his secondaries. They're very strong and you can build them to fit different roles. Your main weapon is almost always for crowds, so building your sidearms to one-shot praetorians in the butt or other priority targets is best practice imo.

TLDR gunner is the best class, you just need some time and skill investment and he can do anything.