I would buy a "Greyhack" from Gygax and Kuntz without question. Why would I do this, when I do not play Hackmaster?
Because of the devious well of ideas from the fertile imaginations of these two "grandpappys of Rat Bastard DM's" everywhere.
Gary and Rob I believe would not do it unless they could remain fairly true to the source material, and Hackmaster being what it is, they can do this. Monster stats can be replaced as needed by d20 counterparts. Traps can be "d20-ized" as needed. The important thing to mine from a module is not the exact verbatim statistics; it is the ideas, and the concept that Gary and rob finally got out a truer representation of this great original dungeon than what has ever been presented before.
If this is true, and if it does happen, I would buy it in a heartbeat.
Let me also go on the record for saying that I loved Ruins of Greyhawk, for it's huge underground empires and its opportunities for being a long-running campaign in and of itself. Contrary to the opinions of its detractors, I found a fairly well-written opus there. It wasn't Gary's Castle, but it was still a well-done work.