I think Bitcoin gambling is illegal because even Bitcoin isn't accepted in many country, so using them with a purpose is totally illegal
Ya their are many countries where gambling is illegal so definitely if you gamble through any source of medium its illegal. Also even if gambling is legal but bitcoin is not then too using btc for the gambling become illegal. So it will definitely depend upon the country and its polices towards it. May be better to avoid if some is not sure about both the aspects of it.
In most counties bitcoin gambling will be legal because bitcoin is decentralized and the governments have nothing to do with bitcoin. In my country gambling is not allowed but this action does not work in case of bitcoin because the government has no control over bitcoin and people can easily use their bitcoin in gambling. But I want to say that gambling is not good and should not lose his hard earn money in gambling.