The GX50 is something of an oddity in some ways. I actually make a
fair amount of money just showing people how to set the OBS in that
box. It doesnt work like the others--wont read it from the external
display, you have to set it on the keyboard, and they hid it real good,
both on the box and in the manual.

For any GPS to be IFR approach approved, it has to drive an external
CDI indicator. In most installations, it shares the NAV1 indicator
with the #1 VOR receiver, and there is a switching unit on the panel
(somewhere), external to either the GPS or the indicator that controls
which box gets to talk to the display.

Some of the GX50 boxes came with their own, third display which is
dedicated to the GPS, so in these installations there is no NAV/GPS
switch.

The Garmin 430/530 boxes incorporate the #1 VOR rx and the GPS into the
same chassis, so on these boxes the NAV/GPS switch is a pushbutton on
the front of the box, rather than on an external panel. They also
renamed it: the button is labelled CDI, and the annunciator it controls
toggles between VLOC and GPS.

It helps to sit down with your avionics tech and make a block diagram
for your installation so you know how all this is hooked up--in *your*
airplane.

Think you got problems, consider the installation in a high-performance
single or twin, typically equipped with GPS, autopilot and DME:

NAV/GPS switch determines which box (VOR#1 or GPS) drives #1 CDI.
NAV1/NAV2 switch determines which CDI the autopilot follows.
Another NAV1/NAV2 switch determines which VOR rx is used by the DME for
remote automatic channeling.

These three switches are similar enough in appearance and function that
a lot of pilots get really screwed up. Add to this some faded out
labelling and you have a real opportunity to make a complete mess of
things. (Why is that guy in the F15 flying so close to me and waving?)
Gene