Gus is a bookie, which means he stands to make a lot of money if he knows the outcome of a ball game before it is played. Which is why he makes some unsavory proposals to Roy about not playing his best in exchange for a lot of money. He's the third leg in the novel's axis of evil, along with Memo and the Judge. We'd add Max to the list, but he's just a pain in the butt.
Gus is also Roy's rival for Memo's attention. He and Roy sniff around each other like two dogs whenever they find the other one at Memo's place or out at a restaurant with her.
Gus is smart, and knows that Roy's weakness is his insecurity about being poor. He calls him out on it, and knows that the way to manipulate him is through Memo. He's with the Judge at the end of the novel, so we know he was in on the fix. He probably told the judge all he needed to know about Roy to manipulate him into throwing the game.