From VikingWiki

Displaying maps in the Garmin image format

There's a library in development to display maps from Garmin: libgarmin. Not sure if the library's development will continue in the own sourceforge project as it is integrated in Navit, an open source car navigation system, which uses gtk/sdk/opengl to display maps. Yes it will. The point of creating it as separate project was exactly to be used from other projects, so the know how for the maps can be collected in one place. Navit just happens to be the first user of the lib.

Unfortunately the garmin maps are not free, but owners of the more expensive Garmin devices usually have them, too. Fortunately there are a lot of free garmin maps out there.