You can import a TIF picture file into Google Earth and geo-reference the corners, to give the correct scale. Then, draw track lines over the contours, and export them as .kml files, which can be converted to .gpx files with GPSBabel.
That's my current method of choice, anyway. Sigh.
If they would just provide a geo-referencing utility in HumminbirdPC, we could all make our own maps pretty easily, from a multitude of sources. I still fail to understand why they have not done so, since adequate maps are really not available for the vast majority of the world, and these products clearly have an international market.