I didn't know they did a 16th one...and on the Sind, no less! I can't wait to check out.
Bruce Heard wanted to do a Gazetteer-like book for places like Wendar, Heldann, and Sind. There is a long story behind it but I'll short cut it by saying that when the D&D line was cancelled in favor of migrating Mystara over to AD&D 2e there was a final RC-line product in production on the Princess Arc (Champions of Mystara box set).
In CoM were going to be one book on creating skyships and a second book consolidating the Princess Arc articles from Dragon. So TSR's RC-people, knowing RC-D&D was headed for the dustbin and guessing that the AD&D people taking over Mystara would never develop the world outside the Known World region for years, if ever, decided to include a third book having nothing to do with skyships or the Princess Arc that included Gazetteer-like info on the Great Waste and Serpent Peninsula regions of the world . . . Sind receiving the bulk of coverage. Also covered in the third book, albeit very lightly, were Yavdlom, Ulimwigu, and Graakhalia. Quite a few fans call the third CoM book, The Explorer's Manual, the hidden or missing 16th GAZ.
In case you never knew, the Hollow World line received three Gazetteers of its own. Not given the GAZ tag, HWR1, HWR2, and HWR3 were Gazetteers in all but tag. The production style is identical to the GAZ line. . . Hex map poster, DM section Player's section, etc. The Hollow World line revists historical nations from the Outer World past. HWR1 is on Azca, HWR2 is on Nithia, HWR3 is on the Milenian Empire. FWIW, the third part of the HWA line of adventures has info on the people and cultures of ancient Sind.