Authorize.net and Power Store are the only two gateways that are supported by power store.
If you own eCart, it is possible to use that to add cehckout using one of the other gateways that ecart offers, but this is not an easy or supported modification.
if you dont own eCart or want to use a gateway that is not offered by eCart, then you would need to hand code the logic to process the transaction through that gateway.