The GPSBabel reads, writes, and manipulates GPS waypoints in a variety of formats on your Mac. It's a highly flexible GPS tool supporting over 100 supported formats including GPX, Magellan and Garmin.
GPSBabel is useful if you've created waypoints or defined coordinates in any program but need to convert them to be read in another GPS device. This enables you to transfer waypoints from one program to another and it supports the KML format that works with maps drawn in Google Earth.
If you know you're way around a GPS device that it shouldn't take you too long to get to grips with GPSBabel. However, you will need to know how to do at least two things - read data from a file, and write it to another file. There are four basic options you need to know how to use to do these things all of which are explained on the developer Quick Use Guide.
If you know what you're doing with GPS coordinates, then you'll probably find GPSBabel the perfect free solution. If you don't, then you'll need to invest some time learning the basic of GPS and the various formats it comes in.