If I had enough damage to K.O. Luxray from AR on it, but had an expert belt attached, and used Gadget Bolt to discard expert belt, would oppenent get 2 prizes or one?
If you're saying that Luxray had 12/13 damage counters, but also had an Expert Belt?
You have to finish every action of an attack before prizes. In this case your opponent takes 1 Prize Card, because at the time of the knockout check, the Expert Belt has been discarded.