According to Polymarket, they're a Prediction market, not gambling. It's only gambling in the sense that trading commodity futures is gambling. At least that's my understanding of Polymarket's position.

They can call it what they want, but it's still a betting site - because most of their volume is on sports, and colloquially paying money that will yield a set return on the outcome of a sporting event but be kept by the counterparty if that sporting event outcome does not occur is traditionally called a "bet," rather than a futures contract or "swap" or "contingent investment" or any other phrase that is used to describe a similarly structured contract in other contexts.

Where I was pushing back upthread was the idea that this is "bogus" in some way. Sportsbooks are clearly betting sites, and they are also extremely efficient at distilling information into a prediction on sporting event outcomes. A large liquid sportsbook proposition - like the betting on the winner of an NFL game - is really close to the weak EMH in action, and has an outstanding historic record of being correct (meaning the outcomes against the closing spread are almost exactly 50/50 over time).

"Prediction markets" don't always have that feature. They're not always deep or liquid, sometimes they're propositions on things that literally can't be predicted (which sportsbooks have, too, like the Coin Toss prop bets for the Super Bowl), etc. But for heavily wagered propositions, the idea is that they will function similar to betting lines, and provide a better (though never perfectly accurate) sense of what might happen in a given event.