is it possible to track the payment transactions in Marketo?
What would an integration consist of if it didn't include transactions?
And do you truly mean payment gateway or do you actually mean billing system (i.e. an app that sits above the banking level and manages one or more gateways)?
Anyway, the broad answer is: yes. There's no officially supported payment/billing/banking integration (no "top-tier" integration if you will) but you can plug in, for example, Chargify or PayPal via custom development, or other apps that variously abstract the payment process. You should explain exactly what your requirements are, however. This is not something to spec out in a one-line Community question and then end up down a hard road because someone just said, "Sure."