your assumption of them using similar systems is wrong since the games are completely different engines done by different people (Nintendo didn't code anything in GX. it was all Sega's work and Nintendo only did supervision and management work)

beyond that no one has figured out how this works yet.

 Well different engines often do the same or similar tasks roughly the same way. The code might be different but the task it needs to perform is very similar and by extension probably what the code is actually doing in broad strokes. There are only so many ways to solve a problem.

The only thing I know for certain is that X, and probably GX, uses splines for track generation and I suspect that the closest location on that spline to your vehicle sets the gravity up to a certain distance. But that's just a hunch.