Ben Nieborg's Polish stamp site has an excellent section on Generalgouvernement stamps to give you a bearing on what you've got, just scroll down on the left until you come to the link.
http://www.stampspoland.nl/There were masses of GG stamps produced for the German market. Even at the height of the war the printers in Vienna were pumping out material for philatelic consumption. All the commem. stamps had surcharges which we believe went straight into Hitlers coffers. Most of the regular stamps issued sell for handling charge prices although I constantly see
ebay sellers asking $10 for a 30 cent stamp because it's "old", "rare war-time", or "nazi-era".
Here's one of my fave pieces. A registered postcard bearing a 10gr stamp from the 2nd overprinted set of 1940 plus 2 from the buildings definitive set of 1940. From Warsaw to Jackson Heights, NY, USA. Dated Warsaw 5 Nov 1941, backstamped New York 17 Dec 1941.

This card was sent when Germany & the US were at peace but was delivered when they were at war. The attack on Pearl Harbor happened on Dec 7 1941 & Germany declared war on the US on Dec 11, so even though this was mail from a belligerent country when it arrived on the 17th, it still went through.