My understanding is that WAAS is just a form of differential GPS (DGPS). An additional beacon is used to improve the accuracy (especially altitude accuracy) of GPS. Surveyors, farmers and crop dusters have been doing this for about 15 years.
GBAS is another form of DGPS, but uses ground based ones instead of geo stationary satellites. I have read that some countries use ADF infrastructure (buildings, power supplies and antennae) for GBAS beacon installation and that if you already have all the bricks & mortar parts, the electronic box for GBAS is relatively cheap.
I had always thought that a network of beacons would be an alternative for Australia to launching a satellite. Its basically what AMSA has done for marine GPS. AMSA have 16 land based DGPS beacons that I always thought provided WAAS type accuracy for the marine equivalent of the aviation J routes.
However, some browsing I just did on the FAA GBAS office site suggests that GBAS requires different GPS units in the aircraft. So, now I'm unsure.