Advantages of GT2 over a rack
With a belt system, the belt engages roughly half the pulley. This, and the tension in the belt, ensures the belt always engages the pulley tightly. A belt and pulley system is thus relatively forgiving.

With a rack and pinion system, only a few teeth engage at any given time. To avoid backlash and get the same kind of "tight" engagement, both the gear and the rack need to be made with very high precision. The carriage also needs to be very well constrained, because any wobble of the rack relative to the gear introduces backlash (or binding). Moreover, you also need to keep the rack and pinion well lubricated lest they wear out prematurely.

Given that belt and pulley work well enough, I don't see why you'd need to move to rack and pinion. The main advantage of rack and pinion is that the rack isn't elastic. The maximum length of a belt system is limited by its elasticity, but given we aren't building meter-long 3D printers anyway, that advantage of rack and pinion does not apply.