If you fly at high altitudes in thinner air the True Airspeed (TAS) must be higher than in denser air to get the same best glide ratio. GPS on the other hand always displays the correct groundspeed. -- 'Can I just fly with the same speed for best glide at all altitudes?' -- 'No, but if at higher altitudes you hold your control bar in the same position as at best glide in dense air (lower altitudes) you will get the same best glide despite your increased speed and sink rate.'
This is basically right but only if you fly with a pitot-tube system, like the Galileo, which measures Indicated Airspeed.
When using a propeller-driven sender, like the IQ series varios, the unit always displays True Airspeed (TAS). In this case there is no difference between groundspeed and TAS when there is no wind, no matter how high you fly. For final glide calculations we have to use TAS. TAS has one disadvantage, though: The displayed stall speed changes with altitude. The vario shows true rates of sink or climb, they are compenstated according to ICAO standard atmosphere.