As of right now, sometimes Gun Strike works like a charm, and sometimes straight-out kills you. You can't decide not to perform Gun Strike if you click in its direction, and doing so in a wrong moment can lead to death. 

Sometimes if you start the Gun Strike too late, that very same enemy can have enough time to strike you before you shoot, rendering the attempt suicidal in some cases. Surely I don't have to explain that it feels awful. It's a design flaw that makes the gameplay frustrating if you're unlucky. Not to mention, other enemies can damage you while performing the Gun Strike, too. Some enemy attacks are faster than current Gun Strike animation or almost as fast, so it's not a "skill issue" if you start the Gun Strike on enemy A, and get hit by enemy B during it. I strongly believe no mechanic should ever feel as if it is unfairly punishing the player for a mistake they didn't make, unless using the mechanic or a tool itself supposedly is the mistake.

Gun strike SHOULDN'T be given invulnerability during animation, as that'd make it way too strong.

Instead, it should just be 50% faster and allow you to sidestep during it in a direction of your choice, by pressing the dodge button, which would give you normal dodge i-frames.
To balance it out - the time frame for the Gun Strike (how long does the red target mark linger for) could be shortened or performing the Gun Strike could remove all other active red target marks. 

This should greatly improve the flow and feel of combat yet not make Gun Strike OP: once you have the opportunity to perform a Gun Strike you do it and sidestep if needed, almost instantly going back to fighting, instead of being interrupted by a long animation that you can't cancel, which sometimes even kills you if started in an unfortunate moment.