Just making sure I've got this right.

If a ship with gunner or Luke scores a single crit against a defender with Determination - and determination allows the defender to discard the crit, then Gunner does not trigger because the attack was technically a hit - i.e there were uncancelled hit or crit results - correct?

Correct. If Gunner/Luke/IG88-B were in a position to trigger, then there would be no card dealt to discard.

You can't claim "it's not a hit" after a damage card has been dealt, regardless of whether or not it gets discarded.

It's a hit!

Negative, It did not go in, just impacted on the surface.