Perhaps this is a controversial stance but I think if praying is part of your gambling process you probably shouldn’t be gambling maybe. You have to remember that gambling is paying for entertainment and while some people use it in an attempt to get rich it doesn’t change the fact that you are paying for entertainment. Leave God out of your debauchery.
It depends on which God they are turning to
I am not an expert on religious practices, but now, as far as I know, this issue is very liberalized and you can pray on any issue and to any God (good or evil). Perhaps this process of "communication" with higher beings for the people who are engaged in these are more important than the very process of entertaining betting.