If you have the cellular model you already have the GPS chipset in there. It will work as an in flight GPS as it stands just using the free google maps that come with it. You do need to download the maps for your intended route first while you are on a wireless network. Just load the route as if you were driving and the moving map will work fine.
Air Nav Pro is free (I think still) but the best by a mile is Sky Demon. That isn't free but it has captured probably 80 % of the UK market at the light end of FW GA.