Charles E. Gannon authored the excellent four-part "Operation Back Door" adventure in issues #49 - 52 of GDW's _Challenge_ Magazine, as well as an additional article on the "Ylii" in #50. Other than a short introduction to the Ylii in the _Kafer Sourcebook_ (which this article contradicts), this is all the info we have on this alien race.
While I have never been able to locate Mr. Gannon, Thomas Vickers has, and Mr. Gannon has provided him with the original version of his adventure, including parts that GDW cut! Thomas has allowed me to link directly to his Challenge Articles page, so you can jump straight there and read them, if you like. Please note that you are leaving my site, when you do, and direct any thanks/comments to Thomas, though!
Still, if you know how to contact Charles E. Gannon, please let me know.