The problem with open source is that it is open for everyone who is interested in it and I think it could be use to cheat the game correct me if i'm wrong.
Ofc, it can, since anyone can see the code, the same with bitcoin, its open source and there are lots of ways to hack it due to its open source. But the thing is, can they afford the resources to do that? Simple answer, no.
Same in gambling as long the payment/withdrawal/deposit code and the probably fair is using secured and good practice code development it will be safe for cheaters and hackers.