What a loss!
Like I'll bet most of us here, I had dozens of his books. He helped nurture my love for this kind of knowledge.
I was privileged to meet him at a conference in the 1980s where he then graciously agreed to come up to my intel unit and give a talk (we could never have afforded him!) for free, on WWII and then current Soviet design philosophy including why the MiG-29 and SU-27 looked so much like the F-14 and F-15. And no, it was not because the Soviets were trying to directly copy US designs.
I still remember driving him around the SF Bay Area that weekend, including going into a hobby/book store I frequented where a good number of his books were for sale. In my mind's eye I can still see people's eyes get big and round when they realized who he was (for months after, whenever I went in there I was a god). He was very proud of what he had done, but so down to earth. Funny thing: with all the involvement he had with aviation, he had never been aboard an American aircraft carrier! There were two in port at Alameda at the time, but given his limited free time we couldn't work it out then. I hope that someday he got his wish.
The world will miss such a prolific (he had a book come out as late as 2010!), opinionated (that's actually a compliment) and knowledgeable writer, and such a decent man.
I'm feeling more than a bit down right now.
R.I.P. Mr. Gunston, and thank you.