Here are two excerpts from the C172 panel.cfg.
To add a GPS you need to add a WindowNN=GPS line in the Window Titles section. Notice how the C172 uses the line "Window02=GPS" to call the GPS. Change the NN to an appropriate number (note that only numbers 00 to 09 will display when press shift+number key). (you did make a copy of the original file first?)
Then copy and paste the whole [Window02] section into the panel.cfg file (preferably in the right order) and change the 02 to the same number you used in [Window Titles]. The GPS should now pop up when you press the appropriate key combo. This section defines how the GPS will display as well as calling the gauge itself.
[Window Titles]
Window00=Main Panel
Window01=Radio Stack
Window02=GPS
Window03=IFR Panel
Window04=Landing View
Window05=Mini Panel
[Window02]
size_mm=456,378
window_size=0.5
position=8
BACKGROUND_COLOR=0,0,0
VISIBLE=0
ident=GPS_PANEL
zorder=4
gauge00=fs9gps!gps_500, 0,0