Using the publicly available database of airman certificates from the
FAA for Jan 2006, and recognizing that some pilots have invoked privacy
and are not in the public list, I find

19014 glider pilots with private glider ratings (though 1420 ratings
based on foreign ratings, a few are duplicated, holding commercial and
CFIG)
12772 glider pilots with a commercial rating
3041 glider pilots with a CFIG rating, however there are 6029 listed
with both commercial AND CFIG, so there are between 9731 and 9767 with
commercial as the highest rating. There are a pilots listed with
foreign private and US commercial to further complicate the queries and
my time is a bit limited to fiddle with this.

Assuming the GAMA numbers are accurate WRT the private only ratings,
about 10% appear to have invoked the privacy option. Assuming that
holds across ratings, I get approximately

19014+9749+3041=31804*1.1=34984 which appears higher than estimates of
five years ago. The number is probably somewhat lower, but in the ball
park.

Frank Whiteley