This is the git repository of the All in one Garmin Map.
http://wiki.openstreetmap.org/wiki/All_in_one_Garmin_Map

This map for mobile Garmin devices is made with mkgmap
http://www.mkgmap.org.uk/

Mkgmap uses special style files to map Openstreetmap features to internal Garmin types.
You can find these Stylefiles used with the aio map here.
There is one for every layer.
The most stuff you will find in the basemap layer style and typ files.

Typfiles define how the special Garmin types are displayed on the device.
You can edit them for example with an online editor:
http://ati.land.cz/gps/typdecomp/editor.cgi


The whole stuff here is public domain. Do what you want to do with it.