Never setup extra keys in gnome ( I am mostly a KDE user ) but found a link that might help.
http://howto.linuxkungfu.org/extra-keys/
If you goto
http://www.google.com/linux and use search items like extra keys, keycode, xev, xbindkeys, and or gnome. Also searching this site had many links as well.
Sorry I could not be of specific help but hope this helps with words to look for in your search. Once you get it please post your solution back
Brian1