I respected Gary's accomplishments, but I didn't know him; I met him once, briefly, when he signed a book for me, and exchanged a few e-mails, and that was it.
Dave I knew, although certainly not well. We met several times, during the course of the work I did on the 3E and 4E updates of Blackmoor--Dave's campaign setting, and the first published setting for D&D--including (briefly) sharing a booth with him at GenCon. While I'm not sure he appreciated my sense of humor, he was always exceedingly friendly and polite, and he really struck me as enjoying his interaction with the fans. Even when his health first took a turn for the worse, he made every effort to attend GenCon in order to remain a part of the community.
He never really got the credit he deserved, as one of D&D's co-creators, and that's a shame--and hopefully a mistake that history will correct as time goes on. But the man had an incredible imagination, and he helped create not only a game, but an entire hobby, that has now outlived him. Like I said, I didn't know him well, but I'm not sure he'd have asked for much more than that.
Rest in peace, Dave. And thank you.
EtA: Some sources are saying that he hasn't passed yet, but that he's in hospice and that it's only a matter of days. If that's the case, while I certainly hope for a miracle recovery, I don't expect one, and all my thoughts still stand. And in either case, my sincerest sympathies to his family and friends.