If you are a league follower, know the stats, who is in form etc, this not a gambling!
Gambling is when you play roulette and anything can happen.
That's not correct because whether you know it or not, betting on sports is also not easier because even underdogs can win sometimes against teams for whom odds are in their favor. So, it's not relevant what you are saying that only roulette and casino is gambling as many have knowledge about these things too, but they still lose and even in sports, the same problem is there.