Other than the raw filesystem reads, which wouldn't help you, all the GPS stuff is done in the frontend software that is closed source.
This source code would only help if you were trying to run the Garmin software from the dedicated device on another ARM based Linux handheld. Similar to what some Zaurus users have done with
TomTom software.
There are multiple opensource software GPS packages out there, some with ability to get maps from several sources, including government maps.