As for adults who have already smoked, it will remain like that.
Maybe it's not related to DNA but maybe how we can choose which one to use and that's more personal.
Seems you're contradicts yourself.
If you said the environment is full of smokers, then you will become smoker too. But you said it's depend on your personal choice, this mean environment do not always make the adult become smoker even he surrounded full of smokers.
But the OP is about the transfer of gambling from parents to kids - which is not possible - because not all the kids become gamblers if the parents are gamblers. However it is also one of the possibility that gamblers kids refuse to gamble incase they dislike gambling.
Don't forget almost all parents often advice to not follow his bad habit or don't become like him at all and teach why they shouldn't become like him, his kids will know why the reason.
I don't understand why most people view gambling as an illegal/criminal/bad thing to do, because if you can control yourself you will know gambling is good for your mind and health due to stressed out of working entire days.