The GD10 is a two terminal gas filled cold cathode device. We have no further details as yet.
Ian Brown sent the images and has the following information.
The GD10 valve was manufactured by Ferranti and used by them in some of their airborne radar equipment, including the AI23 radar used in the English Electric Lightning.
The main image shows a couple of GD10 valves in the circuitry of a Ferranti Blue Parrot radar.
Part of the circuit diagram from a manual for an AI23 radar.
The thin glass tube envelope is mm in diameter, and excluding the base pins is mm tall.
Updated February 21, 2017.