First off I want to say conceptually I love the character but there are some pretty big issues regarding his set up. First off GK, is going to be next to irrelevant in the meta, which given his melee focus is easily understandable, anything in the 120-160 range is near impossible for melee careers to preform well, however his place in the more casual pub meta is still in question.
The lack of ranged in this game is a huge negative, and provides a disservice to your team, forcing them to deal with the majority of specials out of your reach (blights, most assassins, gunners, ect) so the tradeoff for not having that ability should be considerable, however GK is severely lacking on a number of fronts.
- no attack speed buffs
- slow movement
- little damage reduction
- no crit build (outside of his 4x crit talent, however he is forced to basically rely on weapon modifiers and base crit, given his lack of crit talents)
- limited melee arsenal
- RNG duties and their mediocre effects (sans the health regen, which isn’t usable on weaves or FoW, the only content that you would really need it in)
Ultimately his RNG duties are weak, his weapon options are very limiting, he lacks the potential killing speed of some other classes, while bringing little to none of the utility and while his active can be particularly strong, its still less single target than Shade’s (and doesn’t grant stealth) and it leaves you vulnerable in its long animation cycle.
I think a lot of these issues are fairly irrelevant for Legend and below, however, it is still fairly important to point out (now that slayer has a ranged option) that GK, is the only exclusively melee career, and fails to be the best at melee, while also not bringing much to the table outside of damage.
TL;DR Other careers may be over-tuned if we feel like GK is currently balanced, since many outpace him, which becomes more apparently when progressing in difficulty.