Hide Forgot
I'm told this might be a packaging error rather than a Gnome one. Running RH 6.1 with October Gnome, I see two Gimps and XDvi in the Graphics menu and only one (and no XDvi) in the menu editor. Many other items are missing from the menu editor too. I just found all the .desktop files for the 'missing' ones in /etc/X11/applink/*. They're getting picked up by whatever controls the Gnome menus, but not by the menu editor.
I have verified this to be a problem. I am assigning this to a developer for further review.
It's pretty hard to do anything about this, since you'd have to (somehow) in the GUI let the user specify whether each item should live in /etc/X11/applink or in /usr/share/gnome/apps. My best idea for a fix would be adding the ability to switch gmenu from editing one set of menus to adding another. This is probably not worth fixing until the menu merging code becomes a standard part of GNOME (which may wait until /etc/X11/applnk or an alternative is standardized across distributions); the operation of gmenu for a user is unaffected. gmenu p
Since my last comment looks unclear to me, what I meant was that unless you are running gmenu as root (to edit the menus for all users), then this problem won't matter, since normal users can only edit the "User Menus" section.