I have not participated in this specific event, but ride similar terrain all the time here in Utah. All of the above are great comments/suggestions. I would encourage you to take at least the following:
1. Arm warmers
2. Leg warmers
3. Light weight wind jacket, or at least a vest.
4. Plenty of gel and energy bars.
5. And don't forget the sun screen. The sun is really intense at altitude.

If possible, it would be quite helpful if you could spend several days or more at altitude, but that may not be possible. I'm sure you'll really enjoy the event, riding in the mountains can't be beat. I'm envious!