Set the NAV/GPS mode to GPS. I have added the default C172 switch to the 727 A/P to be able to do this.
I made the following changes in the panel.cfg for all the models (100/200/100F/200F):

[Window03]
Background_color=0,0,0
size_mm=447,106
position=2
visible=0
ident=10005
gauge00=Captain_Sim.p721.P12!P12_001_ap, 0,0, 427,106
gauge01=Cessna!Nav_GPS_Annunciator, 0,0,20,20
gauge02=Cessna!Nav GPS Switch, 0,20,20,20

Hope this helps.

Mark