Hello Tricia,
I'm not familiar with this area, but I know that Guben is divided by the German Polish state border, the Neiße river runs along the border.
I believe the main part is located in Germany, north-east of Cottbus. The Poland part called Gubin now.
There is also a website: www.guben.de
The german phone directory (www.teleauskunft.de) shows a lot of Müller (or Mueller) in Guben.
Good luck,
Werner Honkomp