We currently offer a massive range of downloads for FSX, as well as older flight simulator add-ons for the ever-popular FS2004, FS2002, CFS3 and now becoming ever popular with dediced virtual aviators is Laminar Research's X-Plane series.
The GPS switch slaves the NAV button on the MCP primarily to the GPS. If you have a flight plan entered on the GPS and you press NAV on the MCP and have GPS selected on the NAV/GPS button then your aircraft will navigate along the waypoint shown on the GPS.
If you have the button selected to NAV, the MCP is slaved to navigational aids such as a NDB, VOR, ILS. The APP button on the MCP also works off the NAV selection.
In short, GPS is just for following the GPS routing. Any other use can be done through the NAV selection.