If you get the key for free, which you can, but is by far not the most common way most keys are acquired, then probably it would not constitute gambling, as the player isn't putting any money in.

This... is how it originally worked, I think, for a time, when TF2 first came out.

You could get keys from either randomly playing the game and finishing or spending time in a round, or via achievements, iirc.

But yeah, then the keys could also be bought for money, and the whole trading market and steam wallet were introduced, and since then, probably 99% of keys for any game with em are bought, not acquired via in game activity.

You technically can