GroupDeal uses a system known as pre-authorization in order to block the payment for the coupon before it is actually taken. This is because you can use the 'tipping' function built in to GroupDeal where a coupon only becomes active after a certain number of people have bought it.

For example if the deal needs to sell 10 coupons before it becomes active, the first 9 buyers make their payment but the money is only blocked by the credit card company, it is not collected. Once the tenth person makes the purchase, GroupDeal automatically requests the credit card company to process the payment and starts sending out the coupons.

Only certain payment processors have this pre-authorization facility so if you want to integrate an alternative gateway check to see if it supports pre-authorization. If it doesn't, then you will still be able to use the gateway but not take advantage of the tipping point feature of the software.