Naturally, the benefits in terms of getting the result from prayer are exactly zero. But if the player is very nervous and prayer helps him to endure stress more easily, then why not. We must consider this as a kind of meditative practice that helps calm the nerves.
Yes, it will help them directly or not, but it is better to pray and let it happen. Playing gambling with high tension will not be good as we can not enjoy the game and maybe lead us to lose. Hopefully, prayer before gamble can help a gambler to calm down.
I hope so that's the case but as others might say "there will be nothing to lose if you just believe". But as per mention of luck and prayer I think they are in contrast on majority of religions but we aren't talking here on that I'm just reiterating it for context. I still believe it will be luck no matter what unless it really requires startegy.
It might be against most religions, but believe it or not, some people might pray to GOD before they gamble and ask for help to win the games. We do not know how it will impact their game because they will know it later. Yes, in this matter, luck will take part to win the gambling games. The pray can help them to feel confident.