To use gpsbabel and Garmin Forerunner, a command like the one below is used

gpsbabel -t -i garmin -f usb: -o gtrnctr -F /tmp/file.gtrnctr

“usb:” is the input device. The problem is that Pytrainer checks if the device exists as a file/directory, which it doesn’t. The workaround is to open /usr/share/pytrainer/plugins/garmin-hr/main.py and change

if not os.path.exists(options.device):

into

if (not os.path.exists(options.device) and not options.device == "usb:"):

Updated 04-05-08 12:41:
Philippe Piquer just pointed out that the reason it don't work in Ubuntu 8.04, is that the garmin_gps module has been blacklisted. According to /etc/modprobe.d/blacklist because

# most apps now use garmin usb driver directly (Ubuntu: #114565)

Updated 04-05-08 21:21:
There must be a file /etc/udev/rules.d/51-garmin.rules containing

SYSFS{idVendor}=="091e", SYSFS{idProduct}=="0003", GROUP="plugdev", MODE="660"