If you don't have access to Garmins own Mapsource or BaseCamp applications (they are free if you have a garmin mapset installed), then GPS Utility is a good option that will also open GDB files.
Either way you then need to save in a format compatible with MM Import such as CSV, DXF, Shape etc - the capabilities of the apps varies, then in MM import under Utilities / Vector Utilities / Import Files.
If you're stuck I'm happy to do it for you if you email me.
Another way is to import the GDB file into Mapsource trip and waypoint manager (on the CD that comes with Garmin GPS units) or to GPS Utility (but note that the free version is limited to 100 waypoints). Then download the data into your Garmin GPS and upload from the GPS as waypoints directly into Mapmaker in the usual way.