For Info,
If you are doing routing in google maps and want to convert the KMZ format of google (my maps) to GPX. You can use basecamp to do this. In google maps export the mymaps route as a KMZ/KML this can be opened directly by basecamp as a track. If you convert the track directly to a route using the M/Cycle profile it will route you directly to the end of the route if you go off piste unless you turn off auto recalc and just navigate yourself back to the magenta line. You can circumvent this by converting the track to a route using the Direct profile selecting the number of shaping points that you want calculating the route and then recalculating the direct route using the M/C profile. If you then want specific waypoints included at the correct points in the route insert them into the route at the correct point (least likely to alter the route) or include them and optimize the route (can have unintended consequences).