They have a very informative Website:
Among other interesting things, they organise annual gathering of GS & GSA enthusiasts, GS & GSA IG Sommertreffen.
So we drove from Slovenia to Rötgesbüttel, Germany (between Gifhorn and Braunschweig), a little over 1000 km, to attend the GS & GSA IG Sommertreffen 2015.
It was a special meeting, as it was 25th anniversary of GS & GSA IG club, and also 45th anniversary of Citroen GS.
A number of pictures from the meeting can be found here:
And here's the video.