Like ILS stated it is a rather common stamp, the cover seems to be addressed to the area of Poland that was colonized and called the General Government (
http://en.wikipedia.org/wiki/General_Government) by Germany between 1939 and 1945. The "Polenlager" stamp could be translated (via Google Translate) as Poland Camp and it is located on Leithe Street.
Gelsenkirchen was the location of a sub concentration camp in 1944 (
http://en.wikipedia.org/wiki/Gelsen...Third_Reich) so this could also be tied into one of your covers.
The rest of the cover would be helpful in determining the value of it.
Alyn