Gambling certainly isn't dead in RS3, the sand casino is always bustling with people staking enormous amounts of gp. Although i'm just talking about the duel arena as I can't say for any other games of chance for RS3.
One thing I will add though, is that you really do have to be careful with the many different scams that can happen in the arena. There are so many ways to get tricked and people are very deceiving with alternate accounts. I'd just recommend you to tread with caution and make sure you duel safely