@Tensai_Zoo
Like mentioned before, the passive shield not only glows a bright orange to enemy players that may not have seen you otherwise, (It would be less of a problem if you could toggle it without having to aim first...) but it also goes down relatively quickly & more often than not some shots will whizz right over/below it & still manage to hit Gib from his large size alone. Not to mention the effect overlay on your screen while using it, which can be distracting for a good amount of people.
The shield dome is barely useful outside of long-range combat since you can't shoot out of it from inside, making it a game of poking out to take shots for either side, but any decent player can time their shots to wait for the moment to strike. It can even be abused against Gib with rushing Peacekeepers, since you can just take your time to aim, step in slightly to pop a shot, & step out to load in the next shell. Only real use I've seen for it is giving a moment to heal, recharge shields, or revive teammates in sniper fights or such. And like his passive shield, it's a big, glowing, orange dome that will instantly bring the attention of any squads nearby; Especially since it goes through walls.
His ultimate is probably the only real good thing about him, since it's basically a more controlled, instantaneous version of Bangalore's ultimate that can definitely take out a crowd or force a squad to move in your favor. Gibraltar isn't necessarily unusable, but he's definitely got more flaws than strengths right now.