I believe that even if a person has a genetic predisposition to gambling, it is not a particular problem because a person can control his actions and refrain from negative experiences.
In my opinion we choose our own destiny and what to do in life. If a gambler blames his losses on his genes, most likely he is afraid to admit his mistakes and tries to blame someone else for his mistakes.
Even though I agree with you that our genes do not completely write our destiny, we should not understimate the impact they can have if we do notpay attention to them or if we are not aware of them, like most of the cases.
Of course we have to admit that there are genetic traits that are easier to identify than others, if a person's grandfather and father both suffered from diabetes during their adulthood, one can expect the person to have a high chance to go through it as well. In the case gambling as a genetic affinity, it would not be so obvious. In the end, the important thing about a gambling problem is to find support and help to overcome it, whether genetic or not.