The Gamble option is a feature that exists on certain games, which, when you win a payout, allows you to choose to gamble that payout in order to increase it (although as it is a gamble, you may also lose it).
For example: a gamble game will give you a 50-50 chance of doubling the credits you have just won on the reels, by (for example) guessing whether a playing card is red or black. If your gamble is successful, you can opt to gamble again, and continue doing so up to the gamble limit of that game. If you lose a gamble however, you lose all the credits you have just won, including the ones that you just won on the reels.
As well as the 50-50 gamble described above, there may be higher odds bets available, such as picking the exact suit of a card, rather than just the colour.
Only games that have a Gamble button on the Button Bar have a Gamble feature. To activate the Gamble option, click on the Gamble button when you have just won a payout in the main game; at other times, the button will be inactive.
