I pray a lot and I'm also praying when placing a bet or rolling a dice, it doesn't matter if I win or not the most important is I am putting faith in my belief and the almighty, you can pray in any situation any place and any condition, it's beneficial in both body and mind and I don't see anything wrong why you should pray when gambling.
Are you a buddhist?
I've known people on that religion that prays to have good luck as they gamble and they always do that before they gamble. I've seen one how they do it personally so it's not really questionable if you do that.
Not only buddhist pray but there are many other religions who pray to their god. I am a Muslim and i also pray to God as he is the one who have given us life and he controls everything.
However we don't pray for winning in gambling and all such stuff related to gambling / speculation etc.