It is possible to use eCart to modify the checkout process to use a different gateway but it is not supported.
PowerStore was created to work with PayPal Payments Pro, Express Checkout and Authorize.net.
because of the way that some of the other gateway work, using one of the other unsupported gateways may break functionality in PowerStore like order management or stock control.
To add in an unsupported gateway, you would need to edit the confirm page. Remove the existing Local Checkout server behaviors.
Then add the new local checkout server behavior for the payment gateway you wish to use.