I think it depends on how you play it:
For some players it is most assuredly gambling! They take huge risks and try to hit cards.
For elite players who have mastered the theory and practiced all the spots, the edge comes down to performance: who can focus better, and longer, and under higher degrees of pressure. At such a level, it is a sport.
For those who approach it socially, as something to do, it takes on the quality of a game. Or a puzzle, for those who are more reclusive.
That's what's great about it... it can be many things to many different types of people.