A menu item in gnome is stored in a ".desktop" file.
The default system location for them, for all users, is: /usr/share/applications/
I think per user .desktop files are located in: ~/.gnome2/vfolders/applications/
These .desktop files are simple text files. Take a look at some in /usr/share/applications/. You should be able to see from them what you need to put in to get what you want.
This page about gnome menu editing
might also help.
(I am no expert as gnome is not my usual environment, but I hope this helps.)