I have flown many flights in the 727 using the default FS Flight Planner and GPS. You need to either edit the panel.cfg or set a keyboard shortcut for the NAV/GPS Switch (the CS 727s don't come with the switch). My shortcut is CTRL+SHIFT+N. I have also added the default C172 Nav/GPS Switch to the 727 2D Autopilot Panel so you can actually see that it is set to either NAV or GPS (as well as click on it to change between NAV/GPS).

This is the changes I made to all of the 727 panel.cfg's:

Original
[Window03]
Background_color=0,0,0
size_mm=427,106
position=2
visible=0
ident=10005
gauge00=Captain_Sim.p721.P12!P12_001_ap, 0,0, 427,106

My modification
[Window03]
Background_color=0,0,0
size_mm=447,106
position=2
visible=0
ident=10005

gauge00=Captain_Sim.p721.P12!P12_001_ap, 20,0,427,106
gauge01=Cessna!Nav_GPS_Annunciator, 0,0,20,20
gauge02=Cessna!Nav GPS Switch, 20,0,20,20

Backup the original panel.cfg first!

Hope this helps.

Mark