What is the difference between RTK GPS and GBAS?
There are two primary differences between RTK GPS and GBAS, function and implementing technology.  

Functionally, RTK is a differential GPS (DGPS) positioning system. It allows for very precise position measurements (~1 cm). Its primary application is for survey and related measurements.  

GBAS also provides a DGPS function but it adds specialized support for aircraft approach guidance. To that end it provides via data broadcast to the using receiver reference path data for a number of runway approaches. It also provides signal integrity data (e.g. fault detection and alerts) to the user.  

The difference in implementing technology is that the RTK reference station and using receiver determine position by carrier-phase tracking and the GBAS reference station and using receivers use code tracking.