The Collaborative International Dictionary of English v.0.48:

G-spot \G-spot\ n. [from Ernst Gr[aum]fenberg, the gynecologist
   who first described it.]
   A small spot on the front wall of the vagina in women,
   reported to be exceptionally able to produce high sexual
   excitement or orgasm when stimulated. Its existence is not
   universally accepted.

   Syn: Gr[aum]fenberg spot.
        [PJC]