I'm another English Gange trying to find out where we came from. The earliest reliable mention of the Gange family I have found was in the Norway Post. Apparently the Ganges used to be one of Norway's most powerful families; their most well known member, Rolv or Rollo was from the Island of Giske or Vigra. Rolv apparently conquered Normandie in 911 and became an ancestor of William the conqueror. So the French connection isn't too misleading, although the family archives in Chester give two accounts of where our name originated, both of which are German.
Good luck, David Gange.