From Diablo Wiki
Gambling in Diablo 3: Reaper of Souls comes in the form of exchanging Blood Shards, awarded whilst completing Rifts, for a mysterious item. Although the stats of the item are not known the item time can be selected.
There isn't a true element of Gambling because the shards can not be used for anything other than at the merchant, Kadala, for these items. The level cap for Blood Shards if 500, beyond that they will remain on the ground and can not be picked up so they may as well be 'gambled' for these mysterious items. Below is a table detailing the types of items and the number of Blood Shards required to buy one.
While Diablo 3's first Game Director, Jay Wilson, was initially "undecided" on gambling, later in development Blizzard said that gambling, as it worked in Diablo II, would probably not return in Diablo III, since crafting items with the Artisans serves much the same purpose, but better. . 
As far as a system dedicated to gambling, there are no current plans, but as I just said gambling could be pulled off a whole lot better through the crafting system directly. If we thought those types of recipes were a good idea.Which I have no idea if we do. ‘We’ being other people that decide these things
- See the page for full details of Gambling in Diablo 2.
- ↑ Game Director undecided on gambling Jay Wilson, 16/10/2009
- ↑ Gambling system - Blizzard CM, 21/09/2010