Gibraltar shield is to op

It's really not OP. It's strong, but it's not unbeatable. You can still hit him in the head with most weapons even if his shield is up, which is made much easier with optics with a higher zoom OR sniper rifles. In general, you'll want to either get up close and personal with shotguns, force him to drop shield, blitz them so they don't have time to pull up shield, or use cover to constantly peek them. If they're dumb, they'll drop shield when you drop behind cover, simply use that to your advantage as pushing enemies while aiming isn't exactly very fast. Employ standard strafing and dodging tactics, and you should be okay.