[Gmsh] problems with physical groups

[Alexander Sauter]

Dear Sir,

I am testing Gmesh for the IAG at the University Stuttgart. Everything works fine, I am able to construct the test geometry and generate a three dimensional mesh, but I fail at integrating physical entities. They exist in the geo file, and they are shown in the GUI, but they do not appear in the mesh output file. Do you have an tutorial where the use of physical groups is explained? They would be very impotant to specify boudary conditions for our solver. 
Thank you for your Help

Yous sincerely

Alexander Sauter