GPSBabelWrapper gives to the rest of us the possibility to use rich functionality of GPSBabel via familiar Windows interface. It only builds a command line and calls GPSBabel. Hopefully it does it well enough so that you will be spared of typing on the DOS prompt.

Convert file from one format to another
Do the following:

switch Input area to "File" and select file and format. The ">>" button lets you specify format-specific options.
select "waypoints", "tracks" and "routes" checkboxes
select file name and format in the Output area
click "Process" button. The Results window should show "Command succeeded".

View file in QuakeMap
Here is the quick way of seeing the file's contents on the map:

run QuakeMap separately (actually, clicking "Process" button will start it, but start-up delay may be too long for QuakeMap to pick up the data).
switch Input area to "File" and select file and format. The ">>" button lets you specify format-specific options.
select "waypoints", "tracks" and "routes" checkboxes
select QuakeMap in the Output area (aerial view is the most common)
click "Process" button. QuakeMap will show waypoints, routes and tracks as selected.

Read USB-connected GPS data into QuakeMap
run QuakeMap separately
switch Input area to "GPS" and select your GPS type ("Garmin USB").
select only one of "waypoints", "tracks" or "routes" checkboxes
select QuakeMap in the Output area (aerial view is the most common)
click "Process" button. QuakeMap will show waypoints, routes or tracks from GPS.

Write QuakeMap data to USB-connected GPS
run QuakeMap separately. Make sure you have data (wayponts, tracks or routes, for example read a GPX or LOC file in)
select QuakeMap in the Input area
select only one of "waypoints", "tracks" or "routes" checkboxes
switch Output area to "GPS" and select your GPS type ("Garmin USB").
click "Process" button. QuakeMap loads waypoints, routes or tracks to GPS.