If you want to connect your Garmin GPS12 to a computer with only USB ports you’ll need the Garmin serial datacable and a serial to USB converter.
Make sure you get a serial to USB converter with a Prolific chipset, these are supported by Linux.
While the Prolific is supported by Linux, the driver is not automatically loaded, you have to add the next line to the file /etc/modules
usbserial vendor=0×067b product=0×2303
Now install GPSBabel to communicate with your GPS, type in the terminal the next command
sudo apt-get install gpsbabel gpsbabel-gui
This will install the CLI and gpsbabelfe, a QT based frontend for gpsbabel.
To connect to your GPS choose (input) Garmin serial/USB protocol. Now you can download waypoints, routes and tracks from the GPS12 to your computer.
Choose (output) Garmin Mapsource .gdb or GPX XML .gpx
The information about automatically loading the Prolific driver comes from HamSphere NET Blogs