Good question. I'm not sure off the top of my head, but I'd
think that the behavior should be the same in GMATPrep as it is on the official test. That's a little complicated, though, because GMATPrep fairly recently changed to a web-based system. I loaded up an old version of GMATPrep and found that scrolling the mouse wheel up (away from me) scrolled the page up, and scrolling it down (towards me) scrolled the page down.
But if you want a definitive answer, I'd consider shooting an email to the GMAC (
https://www.mba.com/service/contact-us) - or maybe somebody who's taken the test more recently will remember exactly how it behaves.