The least possible!! Seriously. No need for extra gear you won't likely need. Lighten up the bike to a bare minimum. Bring some arm warmers and some light full finger gloves depending on the weather near the top. A cap under the helmet might be nice from the start, but not much else unless you are one to get cold easily. Then, a light jacket or wind vest maybe. One large bottle of fluid should be enough. No need for food and such (a gel packet a most?). Something to fix a flat and that is it I guess.

Best of luck and let us know how it went later on.