Is there an OpenVMS EDT keypad plugin for gedit?
There are none listed at the Gnome Gedit plugin page but that page does have a list of "Requested plugins" - maybe you can get your idea listed there?
I know very little about Gedit and GTK but it appears Gedit should recognise the GTK-key-theme settings. If so, you might be able to develop one that provides the VMS EDT layout by adapting an example such as vim GTK key bindings