No, John, honestly they're not. A GPS device like a Garmin uses GPS coordinates to "plot" your current location on a map, which in the case of a Garmin is either the base map or the full overlay maps on a card. That's all a GPS can do, which is why even to this day commercial airliners, which have had GPS and autopilot capabilities in them for years still use Pitot tubes to calculate their elevation.

Consumer-class GPS devices like a Garmin add the elevation function through use of a barometric altimeter. Were it not for that, a Garmin couldn't give you that info. The URL below is talking about a different Garmin product, but references the use of a b.a. to calculate elevation:

http://garmin.blogs.com/my_weblog/page/24/