I wouldn't call it gambling. That's a tournament, a competition. If it was considered gambling, bloody hell the entirety of the eSports scene would be considered one, the only difference is the prize pool was brought about by various investors instead (afaik). No matter how small the entirety of it is, as long as there's a prize pool, a game based on skill (>99% skill, <1 luck), and players, that's a competition.

If the risk of "losing" money was taken into account, a lot of small-scale tournaments ask for players to pay an entry fee of sorts (based from my experience), but I still wouldn't call that gambling, it's still a tournament. I'd only ever consider an activity as "gambling" if it had that instance of "randomness", bringing luck into the play, and said luck being the majority of the cause for the win/lose. Most competitive types of sports wouldn't really be considered as one, most of them are strategy/skill based.