First space stun, either make it a slow or about half the size, it's the biggest aoe stun in the game... on an out.
The space stun as of now doesn't allow the gunner for any followup and it just makes sure that you can't instantly jump after him or just grip him back in I guess.
Overall minor tweaks are the way to go in any way to balance a bloodline and by completely reworking abilities and throwing random stuff around it'd result in it getting overbuffed or overnerfed.
Sorry for my faulty knowledge on your team btw. (partly why i spoilered it cause I didn't think it was that important)
(btw how does your team do vs mamutz team, seeker being one of the best bloodlines vs gunner)
As to space stun, it can follow up the own blind (situational) but bigger problem is it gives you a ton of damage and a free hit from a teammate as well. It is undodgeable at the same time cause the aoe is too big to walk out of even if you see it.
I suggest those changes because I think gunners playstyle is just not fun and fitting for this game and I know my suggestions are a pretty big rework to a somewhat balanced bloodline but if you fix the space which is just frustrating and there is not reason for it to be that way it would be a good step.
The idea with berta is just another way to make it more fun since e is really boring to deal with as well (you are slowed and even have to wait before you can use any mobility spell to do something again because of spellblock.
Imo it shouldn't be that hard to balance but I agree that this game balance is fragile and can easily tip gunner from slightly op to completely useless or even more op.
I think it is worth it though to improve gunners playstyle in the next few patches and I think my suggestions although very basic are a great change to the game.