Hmmm. I think I may have misunderstood the gunner upgrade cards.
Does Gunner trigger after modifying dice results? I was thinking that it was used if you rolled all blanks or all eyeballs without any focus tokens; now I'm thinking that is wrong.
Nope. You completely resolve the first attack, all the way through spending tokens and removing shields/dealing damage cards. When the attack is complete, Gunner checks if shields have been removed or damage cards have been dealt. If yes, then the ship's turn is over. If not, then Gunner triggers and gives you a new attack.