The ROG G20 uses ducted cooling throughout the entire chassis. A negative-pressure system with airflow from bottom intakes through center fans to top exhaust (and rear GPU slot exhaust). Rising convection airflow is not as significant in this ducted design as fan-powered airflow.

Other mechanical parts will usually operate fine in vertical or horizontal orientations, but there are exceptions. Some HDDs can spin faster and generally last longer in one orientation, some fan bearings wear harder and die sooner in one orientation, and some coolers use heatpipes or vapor chambers which work best in only one orientation. I'm not sure which specific mechanical parts are contained in the G20.

I don't see any other reason to not flip it over. As long as no air intake or exhaust vents are blocked. And as long as no needed devices (like the DVD drive, USB ports, etc) become inaccessible ... how often do you actually use the DVD drive?

Simply try it out - put the G20 on its side, run your favourite heavy game for a little while, see if your temps rise or your performances get throttled.