Is only a start....
As of June 1972;
Chainmail (WM101) - Gary Gygax & Jeff Perren
Tractics (WM102) - Mike Reese, Leon Tucker & Gary Gygax
Hardtack (WM103) - Lou Zocchi ($27 on eBay, 11/04;
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 5931709811 )
Fast Rules for Tabletop Battles Using Miniature WWII Armor, Artillery, & Infantry (WM104) - Mike Reese & Leon Tucker
Don't Give up the Ship (WM105) - Dave Arneson, Gary Gygax & Mike Carr
Ironclad (WM107) - Tom Wham (!) & Don Lowry
Grosstaktik
Wargamer's Guide to Afrika Korps - Don Greenwood ($44 on eBay, 11/04;
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 5931709791 )
Wargamer's Guide to Battle of the Bulge
Wargamer's Guide to Stalingrad
====
EDIT:(*adds in the list found by <Xaxaxe>, below, plus loads more listed at
iSimulacrum.com : Simulacrum on the web *)
Afrika Korps II - 1971 - Chuck Lane
Alexander the Great - 1971 - Gary Gygax
Alexander's Other Battles - 1973 (1972?) - Gary Gygax
Arnhem - 1970
Arsouf - 1969
Atlanta - 1973 - Don Lowry
Atlantis - 1968
Battle of the Atlantic - 1969
Breitenfield - 1970
Cherbourg - 1970
Company Commander - 1968
Corps Cease Afrika Corps - ????
Crusader - 1969
Dunkirk - 1971 - Gary Gygax
Fight in the Skies - 1970 (3ed.) - Michael Carr
Grenadierkriegblitzderweiner - ???? - Jeffrey Jones
Invasion of Sicily - 1972 - Harold Totten
Iwo Jima - 1970
Madrid II - 1968
Metaurus - 1971 - Bruce Belcher
MiG Valley - 1968
Operation Greif - 1970 - Don Lowry et al
Pilsudski - 1969
Poltava - 1970
Second Punic War - 1972 - Jerry Pournelle (!)
Siege of Gondor - 1973 - John Vandevender
Spirit of '76 - 1972 - Chuck Lane
Strategy One - 1967 (2ed) - George Phillies
Victory at Sea - 1972 - Tony Morale
War College - 1971
(Guess that answers my question about Chainmail being their first product!

)
=
Quote from Gary on RPGnet;
The Inside Scoop on Gaming - RPGnet "Guidon Games hired me as its Editor-in-Chief [DW: late 1970] for a new game rules and games line it was about to publish. I put all of the rules mentioned above together into a single book, named it Chainmail,and it was published by Guidon in 1971. As far as I know, Chainmail was the best-selling product that Guidon had. Guidon went out of business in 1974"==
$18 for Ironclad from Noble Knight sounds half-decent, actually...
Noble Knight Games Board and Wargames - Board & War Games (Guidon Games)