On our -400s there is no way of accessing this info via the FMC but I think it may be possible via the CMC/ACMS. I seem to remember getting the GPS time from there a while back (Hrs:min:sec, not the time shown on the POS REF page). Maybe the GPS altitude is in the same group of pages somewhere.
My next flight is on the 5th Jan so I'll have a play and let you know what I find.
Happy new year to all my readers (all 3 of them)!
Eck