Gygax invented another chess game, Fidchell, for the Aerth setting in Dangerous Journeys. It was a shogi-like chess variant, but one of its features were "gates" on the board to the underworld, abyss, land of fey, etc. through which bonus pieces could be summoned.
If that's the one you're interested in, then AFAIK the only complete rules are in Epic of Aerth.