Sorry Terpster, I was being flippant...I should have added some
GBAS is independent of the FMC much like an ILS. If the aircraft didn’t have an FMC (FedEx B-727) you can fly a GLS approach as the data and correction is delivered from the MMR (as it is with an ILS approach).
The FMC offers two benefits to the GLS approach user (as it does the ILS user), ND Map sequencing and missed approach guidance if the missed approach segment is flown in LNAV.
So if you were operating an FMC equipped aircraft on a GLS approach and had a dual FMC failure, it would not impact the approach.