With Diablo 4 Season 4, the Gambling system will receive massive changes, allowing players to gamble on «the best items in the game», according to Blizzard. During their Developer Livestream for Diablo 4's upcoming season, the development team stated that Level 100 characters will receive only Item Level 925 items while gambling.
Season 4: Loot Reborn — Developer Update Summary
Essentially, this means that players will be able to gamble into Legendary items with up to three Greater Affixes, which, besides Unique items with up to four Greater Affixes, will be the best items to wear in the game by far.