It seems that Google changed something, and now the GmaptoGPX output sometimes can't be read by MapSource. But there is a workaround. If you load the file to EasyGPS, then save it as GPX again, then MapSource can load it. Clunky, but least there is a workaround until things get fixed. EasyGPS is available right here (free):
http://www.easygps.com/
and I've also attached a zipped version of the application to this post as well.