Oh, that is long ago, but this question popped up again. Giese is indeed a frequent
German surname, and the main deriviation is from given names lies
Gisbert or Gismar.
See (in
German): Dräger,
Kathrin und Schiller,
Christiane, Giese, in: Digitales Familiennamenwörterbuch Deutschlands,
URL: <
http://www.namenforschung.net/id/name/441/1 >