1. an AI doesn't (yet) have money, and 2. a $20 bribe would not likely persuade a gatekeeper who would presumably be fired from a salaried job for taking it.

Bear in mind that the participants were earnestly interested in settling the question of whether a human could be trusted to step inside the firewall of a potentially dangerous AI. It is reasonable to conclude that they arrived at their decision to let the AI out "in character", and were willing to stick to that decision out of character because their appreciation at having been shown something new outweighed the $10 prize and having to eat crow.

that's against the rules :)

I didn't see a rule against it. Why wouldn't an AI (or Yudkowski) try bribery to assist its escape?

There's a rule about the discussion being between the AI and the Gatekeeper, not between the human behind the AI and the human behind the Gatekeeper.

But in real life, any Gatekeepers will be corruptible humans, no?

the human playing the AI can't bribe the other human in real life. the AI can offer in-roleplay bribes.