Umbran,
I'll throw out a few ideas. I would first ask yourself to define your role as moderator. This sounds easier than it really is. My experience with moderators stems mostly from medical conferences with experts presenting either research, discussing cutting edge treatments, or best practice guidelines. In most of these, the moderators do very little except move the conversation along by asking poignent questions and maybe a few comments. I would define them as being mostly "passive" as they do not present anything directly to the audience. However, they do set the stage by establishing the topic the panel will discuss.
A few suggestions:
1. Figure out who your panelists are. Have them shoot you an BRIEF email on what Gary meant to them. Ask what their role is in the industry. Hopefully they will have given you people with different perspectives. You said you couldn't lean on them, but someone somewhere chose them for a reason. They have to know something. The key is to flesh it out before hand.
2. Establish the format you will have the panelist present in. Is this a round table discussion? Will they prepare some sort of powerpoint slides? Is it a question and answer session?
3. Decide your role...active or passive? If passive you should contact everyone ahead of time and tell them what to expect at the session and what you expect of them?
4. Have you been to this con before? If not, ask them to put you in contact with someone who has and clarify the setup/format.
5. Perhaps contact some of Gary's family, previous WOTC employees, or search old blogs for posts around the time of his passing to see what they think of Gary's legacy.
A few things I think would be cool:
1. A history of Gary presentation....how it all started, his days at TSR, his days after TSR. Many people off the net probably don't know what happened to him after he left TSR.
2. Depending on the expertness of your panel, A debate on Gary's legacy....One side takes the position that he is the founder of DND and the other side is he is the creator of a whole genre of CRPG/literature/art. Mind you this is not to diminish Gary's legacy or in away take away from the man after his death. It would be a discussion of how far do you believe his influence extends. (This topic is a bit difficult to make clear on a post but something comparable to people saying every piece of fantasy lit since Tolkien is a cheap imitation...etc.)
3. Another debate topic: SciFi/Fantasy literature influencing DnD or DnD influencing Fantasy lit.
4. The mysteries of EGG unraveled. What do we know of Gary's personal game. Who played which characters, explain castle Zazyg, etc. What other parts of Gary's game world became Dnd/Greyhawk.
I'll think of more.....just give me a bit.
