I actually don't really see the Kariya comparison. Both were undersized guys with insane college numbers, but Kariya in his prime (basically his years in Anaheim) was an adept goal scorer. He hit 30 four times, 40 twice, and even had a 50 goal season in his career. I know these comparisons are based on play style and not the skill level we expect Gaudreau to reach. But personally I see Gaudreau developing as much more of a playmaker. Even if Johnny does reach his potential, and become as good as we hope he can be, I'd be surprised if he ever hit 30 goals.
I still think the closest comparison is Kane. They both have a certain shiftiness with the puck that practically no one else has. Kane has also been much more of a set-up guy than a finisher in his career. I think that if Gaudreau does end up putting up serious numbers in the NHL, it'll be with his passing ability and vision, and his way of finding guys in perfect spots for scoring chances. Not with his goal-scoring ability, like Kariya.
Note: I'm not saying Kariya couldn't pass the puck either, but his numbers throughout his career saw him put up a close goals-to-assists ratio, especially in his prime. He even had seasons where he put up more goals than assists.