They say that gambling is a zero-sum game. Apparently, no one told u/DaLamb2004 that.
According to the aforementioned Redditor, they've made a killing just playing Blackjack in GTA Online. To be specific, u/DaLamb2004 has made over $35 million and counting. Their strategy? "Always double down", or at least, that's what they seem to imply.
Naturally, other GTA players couldn't help but marvel. Most joked that they end up losing as much if not more, especially if they follow the supposed strategy of doubling down on your bets. Of course, u/DaLamb2004 was likely joking. Blackjack is a lucrative venture if you are of sound mind and can think methodically under pressure. Not to mention, several movies, like "21" from 2008, prove that some people make a living out of playing Blackjack as well as gaming casinos out of their money in real life.

Earning money in GTA Online isn't easy. If you know how to play your cards right (pun intended), you can cut down significantly on the time you spend earning $GTA by gambling.
The introduction of the Diamond Casino & Resort update in 2019 gave players a chance to try out their luck and bet virtual currency in Blackjack and Poker, among several other gambling games. But, fair warning, gambling is addicting.
Keep in mind that, when it comes to casinos, it's the house that always wins. Or, in this case, Rockstar Games.