Here's my quick idea for this.
Slot players = Crankers (cranking that slot machine)
Roulette players = Spinners ('cause well, some of those things spin)
Blackjack players = Jackets (for no specific reason?)
Dice players = 6 siders ('cause a dice has 6 sides?) or Rollers (roll the dice)
Pokers = Yeah, Pokes is a good choice for this one.
Sports Bettors = Managers?
E-Sports Bettors = Keyboards

You've got some good ideas here. For dice players, I would prefer Rollers because the name sounds fancy

Jack or jackers sound more appropriate for blackjack players. I can't stop thinking about my old school jacket, I never like putting it on. Maybe that's why I am not comfortable with that idea.
So I'm curious what nickname we give someone that plays all of these games

Master sounds good to me because Jack of all trades, master of none is likely to be the case.