My understanding is that real-time kinematic (RTK) is also a differential GPS technology to enhance accuracy by using a ground-based station to provide the correction data to the onboard GPS receiver. Is it similar to GBAS? Then what is the difference between RTK 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.
My understanding is that RTK is a specific implementation of the more general GBAS (Ground Based Augmentation System) principle. RTK uses measurements of the phase of the signal's carrier wave in addition to the information content of the signal: https://en.wikipedia.org/wiki/Real-time_kinematic
A summary of different augmentation technologies is here: https://en.wikipedia.org/wiki/GNSS_augmentation