If I reveal a hidden Kennen at a battlefield and my opponent Gusts the Kennen in response to the stun trigger, does the trigger resolve?

No — if Kennen was revealed from Hidden, the stun trigger won't do anything once he's Gusted off the board.

Playing him from Hidden adds a same-battlefield restriction to his play effect ("stun a unit here"), and that "here" is referenced from Kennen himself. With Kennen back in hand, the ability can't confirm the target is at the correct battlefield (rule 359.3.e.12), so the target is illegal and the trigger resolves with no effect — the [2] is still spent.

Worth noting the contrast: if you play Kennen normally from hand, his ability is "stun a unit" with no "here" reference, and Gusting him does not stop the stun from resolving (rule 811.1.d.2, FAQ #12699).