Why would you need to introduce something? Gambling in Chess is usually just $X per game. I guess you could add in side bets or upping the ante, but I've never played that way.
offering odds variation
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one way of introducing gambling to online chess would be to allow a player, at any point in the game, to offer his opponent odds on which of them would win.
i think there should be a limit on how many times a player could do this (as well as a finite range of available odds).
if an opponent declines to take a bet then no money changes hands.
i think this would work well with my recent suggestion about simultaineous chess or indeed any number of alternative variations.