You can configure the programs that gnome runs as the session starts using
/usr/bin/gnome-session-properties. This is a GUI program that should let you re-enable whatever you think you have disabled, using checkboxes. (Assuming you haven't completely removed whatever it was from your system!)
The menu items are little text files that can simply be edited. They have a
.desktop extension. Here is a link to scripting gnome menu entries.
There are also GUI tools for manipulation of the Gnome menu itself. Alacarte is probably the most widely used. This blog entry gives you a quick introduction. You may need to install this tool:
sudo aptitude install alacarte should do it.
Also, if your menu items were created by a previous version of Gnome have a look this forum post, as malformed/outdated entries may exist.