Leadsled said it pretty well.
WAAS requires at least 2 different geosats for coverage, and at least 2 centers.
The ground stations gather the data from the GPS sats. This is all fed to the centers. In the US, there is a center on each coast.
The centers broadcast a hyper-corrected signal up to the 2 geosats, which then broadcast to the coverage area.
You reciever still uses all of the GPS sats, and the WAAS part applies the correction factor.
GBAS is completely different.
A GBAS system is per airport, with at least 3 ant, spaced as far aprt as possible. The GBAS continuously monitors all of the sats avail. The GBAS creates its own correction factor for that location only.
The system broadcasts this correction factor, as well as the code for the final approach for the ac to use. It is a signal broadcast which the ac uses with much the same look alike system as an ILS.
With GBAS, the ac still uses the GPS sats, the GBAS is the correction factor.
Hope that clears it up a bit more.
As noted, there is little hope for WAAS in AUS, but GBAS is a much different story. A typical GBAS unit, installed, is about $3Million per airport. With this you get a max 26 runway ends, and currently cert to Cat I. The signal is accurate enough for CAT III, but it is not cert for it yet. When it does get cert, the system, nor ac will need any mod.
Both Boeing and Airbus offer GBAS at no cost difference on a new ac.