The latest version of ITN Converter (1.76) will open GDB files. However, that file format produces hundreds of navigation nodes for relatively short routes (i.e. a 200 mile Welsh ride-out I organised a month ago has 390 nodes). This causes the ITN Converter editor, which runs off the Google Maps API, to run really slowly.
Best to open the GDB file in Mapsource and save as a GPX file. The same route as above only then included the 18 navigation nodes that I'd added manually to shape the route in Mapsource. This gave the correct route in the ITN Converter editor.
Whenever I post routes I include Garmin GDB, TomTom ITN, generic GPX and Google Maps versions to open it up to as wide an audience as possible. Routes tend to get made up in Mapsource but converted into the other formats (via a 'save as GPX' from Mapsource) using ITN Converter. Maybe you could ask the source of your routes to send you a GPX version, tell him/her to use the 'Save as' option in Mapsource.
Using the example above; Obviously with the full GDB version, sent to the GPS from Mapsource, with all 390 nodes a Garmin GPS should follow the route faithfully. Using the GPX version in a Garmin or ITN version in a TomTom, with only 18 nodes, there may be some anomalies depending on how the routing algorithm differ from Google Maps, the mapping differs and any odd routing preferences set on the device.
ITN Converter is a really great piece of software, I've been using it for years.