Offhand, I'd say very well (ba-dum-tish).
There's a couple things that limit this; usually if Vancian magic is used, you wind up with this idea that you can only win a very small amount; you could gamble but you'd have to try a ton and it would be unwieldy.
If abused, here's some things I'd use as a GM:
Divine disfavor: If the person casting the spell is of Lawful alignment or Good alignment, their god may frown upon rigging gambling.
Divine fallibility: Gods can't be bothered to concern themselves with gambling, and just don't care enough to answer coherently/certainly/accurately.
Hidden error: Technically the spell description says that the caster can know, but if they abuse it they lose this ability (too used to the reactions, don't recognize truthfulness anymore).
Fate gaps: Not everything is guaranteed to happen as divination states; if the match were rigged, for instance, the pit boss may change the outcome at the last second, but when the diviner asked the first person was going to win.
Wards: I'm not familiar with this in d20 (according to Order of the Stick such things definitely exist, and I've read something about similar things but don't recall exactly), but you can get wards to prevent a clear reading/cause a false answer to be apparent.
"Mean" GM (ambiguity): I had a Shadowrun group that depended way too much upon divination, so I let them use it like a GPS to find mission objectives. They didn't know it was hidden in the room with a fake clearly planted, so they took the fake and ran off. This doesn't work so well for sporting matches, but you could focus on a trademark style and then the opponent happens to use it in the next fight, so on and so forth.
Material cost: Thor wants a cut of his cleric's gambling earnings, increasing the Cleric's material cost for Divination/Commune.
Not all of these fit very well in universe, but they're what I use.