Blackjack you will lose the longer you play. It's math. A top DFS player can consistently play profitably.
This is gambling. Not strictly defined as such, but if it looks like a duck, walks like a duck, and quacks like a duck, guess what?
Do you consider betting on horse racing as gambling? To be honest, for the professionals placing bets in this way, I don't (keep in mind it's very very easy to incorrectly place yourself in this category, and you will lose). But the vast majority of people betting on DFS do not, and can not, take this approach. For them, horse racing and DFS are very much gambling.