Gambling in any form is consider to be illegal. But since cryptos are not regulated by any government agency it is free from any accusations with regards to legality issues. Bitcoin has been use to a lot of gambling site here but none of them were restricted by the government and as long as bitcoin is decentralized no reason for it to consider illegal. With the way, so many gambling sites signature campaign has been going in this forum you should have already gotten your answer. And moreover, and the good thing about bitcoin gambling is that no authority can question this actions because of the decentralized nature of bitcoin. And again, user information are highly protected.
Actually, gambling is illegal in my country. Whether it's gambling using Bitcoin or not, but actually Bitcoin has also been banned by my country. So can you imagine how illegal the thing you are talking about right now, hahaha. But we must know that Bitcoin is just an ordinary currency. We can not get Bitcoin law, because Bitcoin is the same as the dollar and other currencies. To be punished is we who use the currency. And it depends on our actions in the currency we use for what activities. So it also depends on the activities we do. If your country and your religion prohibit gambling, then it is illegal.