There used to be an option to enable editable menu accelerators for GNOME apps. The GNOME team removed the GUI for this, but at least under GNOME 2 it was still available via gconf. Recent Ubuntu versions use GNOME 3; I'm not sure of whether that still works (since GNOME 3 has migrated to dconf). I tried it in the old gconf-editor, and setting the option /org/gnome/desktop/interface/can-change-accels using dconf-editor, but it doesn't seem to work in Gedit (v3.4 on Precise).

According to Where to configure shortcut keys of Nautilus?, it doesn't work with Unity's global menu. You could load a different desktop environment and make the change there (if it works).

It may still be possible to edit the keyboard shortcuts by editing configuration files. According to a commenter on the (very outdated) Gedit shortcuts documentation page:

You don't really need a plugin to change keyboard shortcuts. This
(also) works:

~/.config/gedit/accels:

; gedit GtkAccelMap rc-file         -*- scheme -*-

(gtk_accel_path "<Actions>/GeditWindowActions/DocumentsPreviousDocument" "<Control>Page_Up")

(gtk_accel_path "<Actions>/GeditWindowActions/DocumentsNextDocument" "<Control>Page_Down")

(gtk_accel_path "<Actions>/GeditWindowActions/SearchFindPrevious" "<Shift>F3")

(gtk_accel_path "<Actions>/GeditWindowActions/SearchFindNext" "F3")

~/.config/gtk-3.0/gtk.css:

 @binding-set unbind-ctrl-d {

         unbind "<ctrl>d";

         unbind "<shift>F10";

         unbind "<ctrl>Page_Up";

         unbind "<ctrl>Page_Down";

 }

 GtkTreeView { gtk-key-bindings: unbind-ctrl-d; }

 GtkTextView { gtk-key-bindings: unbind-ctrl-d; }

According to one commenter, Ctrl-Tab is hardcoded and cannot be rebound easily, but there is a plugin that purports to do this. To install the plugin, see How do I install a plugin for gEdit v3?.

For gedit2, the plugin files go in ~/.gnome2/gedit/plugins.

If you're unable to get it to work, you could use a different editor that lets you edit shortcuts, such as KDE's Kate.

The list of default keyboard shortcuts in Gedit is available in the manual. Click "Help" > "Contents" > "Shortcut keys" in Gedit to access it.