I think it's not illegal and legal because there are no rules about it.
There are some countries that ban gambling through casinos and even in online site. I do not know why they banned gambling online and offline but we all know they have a good and explanatory reason. Gambling is worst addiction and it can literally destroy lives, if people in a country can't avoid or keep themselves in gambling, then it is better for the government to ban it.
It depends upon if it is prohibited by your own government because obviously all things becomes illegal if it is imposed by the law. If the scenario is that bitcoin gambling will be considered as illegal then can they banned it? maybe they try but its hard for them to do since bitcoin is no central control and there are still gamblers will be anonymously doing that activity.