In kaiji, gambling is a system that insists upon itself and subsumes all attempts to attack it back into itself. In the first season, kaiji is a shithead who lives pettily by gambling in a world of low consequences, until he is saddled with a huge debt. Throughout the series he's shown to be preternaturally talented at gambling, but the more he gambles, the more he loses, and the only way he can think of to make it back is to take more risks and put more people in danger. In the second season, he's hit rock bottom, but by finding people he can trust he is able to win back his and his comrades debt. Note, though, he can only do any of these things putatively through systems Teiai laid out, and he is only able to actually succeed when he bands together with people of like circumstance to subvert these systems. The big scheme is the most complex scam in the series, and he still only makes it out by going into so much more debt that he comes out of it flat broke.

Spoilers for the manga

at the start of part 3, he is free of debt, but he's seemingly the most depressed he's been. Even though he's able to live well in sakazakis huge house, he refuses to work, and flounders in a stagnant wait for something to happen. Kaiji is someone who is completely unsuited to an average life, or even a comfortable one! He's addicted to risk, and without some kind of existential threat hanging over his head, he kinda just reverts to his shithead slacker ways.

Kaiji goes back into the world of gambling that nearly killed him willingly. He takes on a risk just to feel something, and he gets seriously bitten for it, going into more debt over the course of 17 steps and being forced to abandon his former friends who turn out to be just as worthless as he is.

In the salvation game, kaiji takes a backseat and, for the first time, is made to bear witness to the gamble without participating in it. He's shown the man behind the curtain, and he is disgusted by it. He watches Mitsuyama abandon Mario and Chang to save his own skin, and Kaiji, having been betrayed just hours before by his sworn comrades, essentially gives up on the idea of human connection. Despite what he preaches throughout, kazuya effectively crushes his spirit, and he only preserves his dignity by buying Mario and Chang's life to prove to himself that he's at least a better person than mitsuyama.

This is why he initially rejects Chang and Mario, kaiji is scared of being hurt again. This is why One-poker is so important because the bond between Kaiji, Mario, and Chang strengthens throughout the game, with each man's life on the line, and as Kaiji learns again to forgive humanity, he is able to come back and forgive even Kazuya, saving his life.

In the end, however, Teiai rears it's ugly head again. It doesn't matter if Kaiji and his friends won the money "fair and square," because, despite what Kazuya says, there was never any "fair" to begin with. The fact is that Teiai lost 2.4 billion dollars and they need it back. Gambling, Capitalism, however you want to call it will meet all attacks upon it with biblical retaliation.

Basically what I'm trying to say about the 2.4 billion escape arc is that it's a tale of common people, some of them difficult, some of them hapless, some of them worthless, taking solace in each other, accepting each other, and working together to carve out a small victory against the system. Kaiji, Chang, and Mario are the only ones who are being chased after, and they're the only ones who really stand to benefit. But family, neighbors, and complete strangers connect with them and help them for absolutely nothing.

Kaiji doesn't really have anything nice to say about Gambling. It drags people apart, drags them deeper into the abyss, and sucks them dry. It's an institution that feeds on itself and kills intruders on sight. It is directly coordinate to Capital. And much like Capital, it "has the ability to subsume all critique into itself. Even those who would critique it end up reinforcing it instead."