With GBAS, do you refer to Ground-Based Augmentation System (GBAS),[1] if so there is nothing like it in FlightGear. It seems like its main purpose is to make an integrity check of the GNSS[2] position, though it also improves the accuracy slightly.
I guess using Nasal and property rules you could implement a feature emulating varying GNSS accuracy and any jamming,[3] received position, GBAS differential corrections, any GBAS integrity warnings etc. as a separate property subtree. You will likely need to modify the aircraft you are using to use position from the GNSS/GBAS subtree instead of the FDM driven position (that likely can not be overwritten).
Edit: I kind of think that real life aviation and communication is starting to be too dependent on satellites. A severe coronal mass ejection and solar storm, like those of 1859, 1972, 1989 and 2012 (that was not directed toward the Earth) would likely cause problems, quite possibly severe problems.[4]
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[1] See the European Space Agency's wiki Navipedia articles Ground-Based Augmentation System (GBAS) (permalink) and GBAS Fundamentals (perm).
[2] Global navigation satellite system. In essence any or all of GPS, Glonass, Beidou, Gallileo, etc.
[3] There have been issues with people using GPS jammers to not have their position known. This causes problems near runway approaches. See for example GPS Privacy Jammers and RFI at Newark, FAA, March 2011 (PDF, 0.4 MB)
[4] See the Wikipedia articles Solar storm of 1859 (perm), Solar storm of August 1972 (perm), March 1989 geomagnetic storm (perm), Solar storm of 2012 (perm).