I don't think it's reading GPS Speed but rather GPS Altitude.
Speed is on the left side of the ADI and is replaced by a coloured tape indication.
On the right side we usually find altitude information and my guess is the 000 is sea level and the 010 is a thounsand feet with the 520 being mid way on that scale so it's a 520 foot GPS Altitude read out.
Edited... Just noticed is actually says GPS ALT at the bottom of the scale so my assumption is correct!