Access GET data from URL returned after credit card payment

I have set up a simple payment button on my site that re-directs users to a secure payment gateway for my Merchant Account. The gateway then returns to a URL, that I supply, with GET data appended to the URL including data such as transaction number, if payment was accepted or declined, and reason for decline etc. Seems like a different URL for accepted or declined payment would be better, but that is what I have.

I would like to have a page that would display either “successful payment” or “payment declined” depending on the returned information.

How can I display different text (or a different page) depending on the response?

I am new at WordPress and have very limited PHP knowledge. Any help would be appreciated.