You know you can just head shot him.... hes not hard to miss especially at mid to long range if you don't have potato aim.
I play Gibraltar and here is a few things I personally think about him and apparently "OP" ability because obviously he's not.
1). The shield only takes 50 damage and does not cover his head or feet
2). His ADS shield takes about a second to actually activate its not instant...
3). His passive makes him stand out across the map making him impossible not to see
Apex is a very fast paced game so having a big hit box is already a huge disadvantage, if you run into a Gibraltar with his ADS already set up i don't want to be that guy but that's on you because I've never had someone just sit there with and and trade blows it's all about weaving back and fourth rushing into combat. The best thing about him is mortar strike but that is honestly it, weaving in and out of his dome shield is much more effective than ADS and even then a good enemy can just use it against you as well.
I personally feel like hes one of the weakest legends in the game and really needs a good team with good communication to do decently.