If Gastly with the "Swelling Spite" Ability is my last Pokemon in play and is Knocked Out, do I get to search for 2 Haunter and put them on my Bench or do I lose the game?

Yes, you get to use the Ability before you lose. Gastly is Knocked Out, the opponent takes a Prize card, and then you search your deck for 2 Haunter and put them onto your Bench. (Then you would have to promote one of the Haunter as your new Active Pokemon.)

Relevant cards

SM10_67