At the risk of a hanging and in an effort to circle back to the original question ... a recap;

* Most carbon fiber (CF) gliders have a fiberglass (RF transparent) section in the turtledeck. Evidence, picture on the Schelicher page and 27 manual.. YMMV.
* Yes, this glass area can be used for GPS antennas as they are receiving signals from overhead satellites. Think flight recorders. The GPS antenna should be mounted on the surface of the turtledeck for best coverage (not lower on the shelf which will limit receiving signals from GPS satellites nearer to the horizon).
* Antennas that radiate/receive horizontally (FLARM, Xponder, etc) should not be mounted here as the remainder of the CF fuselage will block much of the signal.

May the rebuttals commence.

- John Ω