15% Damage reduced is TOO MUCH for someguy who can tank 50 more damage when aiming at you.
An R-99 deals 9 damage per hit to him or Caustic, which means you need 21 bullets to kill a Gibraltar with a Lvl 3 Body shield if he doesn't use his 50 HP Shield. But in most cases, he will. I don't think the buff applies to his shield so that makes arround 3-4 more bullets to down him.
R-99 with a Lvl 3 Extended Light Mag lets you shoot 30 bullets per mag.
You may need 25 bullets to kill a Gibraltar by yourself.
Now, you're likely to NEVER hit ALL of the bullets in your magazine except if your name is Shroud or NRG Aceu, and he can still tank more bullets with his shield.
In a 1v1 situation, Gibraltar TOTALLY has the advantage. I understand his hitbox is THICC, but it has come to a point where even when landing all of your bullets, you still end up dying because you need to reload and he doesn't.