Investment and gambling is different, also the word "gamble" is a very broad word to begin with. I guess the right term to use when you're using your money in an investment is "risking" and not "gambling" your money because you put it in a game to earn money.

The other way to determine the difference between NFT games and gambling is that NFT games assures you to earn money once you started while in gambling, it is the complete opposite.