You're able to gamble in game currency as long as there's no real world gain from it no matter what. Zenga Poker, Poker Stars, GTA V and their Casino. It's all legit, full on gambling, but there's no real world benefit so it's not technically illegal.
Edit: Some countries may ban the gambling feature like they did Grand Theft Auto's, but there's no legal action they can take on the game for including such a mechanism. Only reason I can see that would give them justification to ban it is the fact that you can buy GTA currency with real world money.