Luis,
I would recommend researching the available direct deposit protocols interfaces. Direct Deposits are usually (in the US at least) handled through Automatic Clearing Houses (ACH).
I haven't looked into them myself but I can tell you that they are different that Credit Card transactions. Which means that you can not use the payment-gateway for direct deposit transactions. The payment-gateway is a good package to explore to get an idea of how you could define an gateway for direct deposit transactions and subsequently write an implementation for the ACH you'll be working with.
/Bart
