A GDO is not a ‘frequency meter’ as such – that term (or the term ‘frequency counter’) is normally used for devices that provide a digital readout of frequency (‘Digital Frequency Meter’, or ‘DFM’). The Grid Dip Oscillator (GDO) is a simple analogue device that pre-dates a digital frequency meter by several decades. I find a GDO to be useful for certain things in radio receiver maintenance. For example, I have used one to determine the resonant frequency of a tuned circuit (actually an IF transformer/capacitor combo) in a circuit having the tuned circuit un-powered, or use the same instrucment ‘Absorption Wavemeter’ mode when the circuit containing the tuned circuit who’s resonant fequency needs to be determined is powered. A GDO can even be used as a simple signal generator or tracer. They have the advantages of verstaility, low cost, ruggedness and ease of operation. They have limited accuracy however, though if you also have an accurate signal generator/DFM, this can be overcome.