If you send it over an NMEA link, OpenCPN has a checkbox on the export dialog box to send in "Garmin" format - resolves a few oddities in Garmin's interpretation of the NMEA "standard"
Don't know about the 72H but I have an old Garmin 128 to which I export routes from OpenCPN on a regular basis.
As said above, use the Garmin setting in OpenCPN (Spanner/GPS/GRMN to GRMN). You may also need to change the "interface" setting on the GPS to GRMN/GRMN depending what your default is. I use the GPS to communicate with the Autohelm so NMEA/NMEA is the default. If you don't export NMEA from the Garmin then you could leave GRMN/GRMN as your default.