The GDO was one of the most important instruments used by amateur radio designers. It can be used to measure  testing resonant circuits and oscillators. In addition can be measure a signal frequency,

the inductivity of coils, the electrical length of coaxial cables, and the resonant frequency of antennas. It can be used both as a signal generator and as a radio receiver. It has a wide range of uses.

A GDO is basically a wide-range oscillator with a meter in the grid-return line to indicate when the resonant frequency (of the tuned circuit it is loosely coupled to) matches the frequency of the GDO oscillator. This GDO uses a EC92 tube in a Colpitts oscillator circuit. The included tube works fine and I will add a new Tungsram EC92 tube as a spare. 

Please note that this is a very old instrument, a collector's item! It works, but the scale is no longer accurate.