Here are my thoughts:
One definition of gambling I found is:
Gambling is the wagering of money or something of material value on an event with an uncertain outcome with the primary intent of winning additional money and/or material goods. Typically, the outcome of the wager is evident within a short period.
By this definition all poker players are gamblers and all casinos are gambling against their customers. Because of that I don't like the definition.
I would define gambling as the act of wagering on an event where the individual or entity wagering is at a disadvantage (has negative expected value) or is at risk of losing all of their money if they were to continue making this wager.
By my definition, a casino is not gambling against the vast majority of their customers. It's evident casinos are profitable businesses largely due to gaming. As long as the casino sets table limits that ensure no risk of ruin, they are guaranteed to make money as long as people with negative expectation keep wagering.
Similarly, a good poker player has an edge against the vast majority of their opponents. If they play within their bankroll, at limits that afford them no chance of going broke, against these opponents, they are identical to a casino. They can lose on any given day, week, or even month just like a casino but given enough time they will assuredly win. When you're guaranteed to win over time with zero risk of ruin, that ain't gambling as far I'm concerned.
So back to the question of "Is poker gambling?" For the players that have negative expectation, yes. For the players with insufficient bankrolls in relation to the limits they're playing, yes. For the players that have an edge and are playing limits that ensure they have no chance of going broke, no.