I just checked this out on the default b737_400 panel to satify myself. I swapped the GPS & Radio stack windows in Panel.cfg & tried various ident= names. RollerBall is correct. The default simicons refer to the ident= name. The [Windowxx] number is not important providing the window entries are in order & the numbers run in sequence. This entry works for me.

[Window01]
size_mm=150,250
position=8
BACKGROUND_COLOR=16,16,16
VISIBLE=0
ident=GPS_PANEL
gauge00=gps!gps,0,0,150,250

PS. The 3rd party switches I've tried use the [Windowxx] numbers. This is what confused me.