A quite interesting paper about construction, frecuencies, history and schemes on GHG "
Some hardly known aspects of the GHG, the U-boat’s group listening
apparatus" by Arthur. O. Bauer, Having consulted to ex GHG operators and so.
Give it a read ! 
here
http://www.cdvandt.org/GHG1996.pdf
Cheers
P.S., btw this document comes from
Foundation for German communication and related technologies(History of Technology)
http://www.cdvandt.org/
where scrolling down you will see a miriad of interesting documents on radar, radio, sonar and the like, many from WW2 era, and some referring U-boat
Enjoy! 