Well, I'm not sure how you know what the A310 mission system technical spec. actually is, vascodegama.... It has moved on considerably since the version you saw.
Manual route entry, editing and manual fuel on board inputs are all error prone - the air refuelling operator shouldn't be required to enter 5 or 6 digit values at whatever granularity would be required to maintain an accurate fuel plan. Way back in Vulcan bomber days, we did a half-hourly fuel check to maintain the howgozit, whereas in the A310 it is done automatically at 1 Hz.
With just GPS, you would only have ground speed and track, plus geo altitude. Somewhat limiting, to my mind.
Party Animal, yes, the A310 system could certainly be operated and interpreted by a commissioned fuse-changer.