But i think even if girlfriend is loyal she will never support her boyfriend gambling habit, because mostly people’s treating it’s a very bad thing and it’s kind of criminal offense. On the other hand wife and your family members will go against it very hardly, because they know that here you can lose money, so they dislike it and they want to protect you from big losses.
Yes, you are correct as it is very rare to see a wife or girlfriend encouraging her husband or boyfriend to gamble. The only exception I think can be something like the girlfriend is a gambler too and therefore she don't mind that his boyfriend also gambles.
Another reason can be some sort of motivation for the GF to allow her BF gambling. Maybe she has known someone who is gambler and he has made a lot of money. So she also hopes that if her husband will gamble, they can change their fortune. Such are unrealistic dreams that rarely come true.