Keep in mind that MOVs will clamp at a level that represents a significant overvoltage relative to the rated maximum voltage, while the GDT will essentially short and clamp to a very low voltage. If the downstream components can absorb a little energy and withstand the front of the transient until the GDT can start conducting, the GDT will actually do a better job controlling the overvoltage.
Zener (avalanche) diodes are very effective and fast but GDTs have the advantage of zero leakage which is important in high impedance circuits.
It is also important to *not* clamp a high impedance input in such a way as to produce a low impedance input because the high current will destroy the protection circuit which still counts as a failure. So a GDT cannot just be placed across the input.
So the universal protection circuit comes down to using a series resistance to limit the current before low leakage shunt protection. Usually the shunt protection uses low leakage diodes which are themselves clamped with a zener (avalanche) diode or other source of low impedance; this has the effect of isolating the zener (avalanche) diode's leakage from the circuit. More protection is provided by using a series resistor followed by a GDT followed by another lower series resistance and then low leakage diode shunt protection. The diode shunts never see any voltage higher than the GDT breakdown voltage so there series resistance is selected accordingly.
If the circuit to be protected can sustain the breakdown voltage of a GDT without damage, then only the series resistance and GDT are needed but this is rare these days. In the past with vacuum tube input stages, the series resistance and GDT in the form of a neon lamp was all that was necessary.
Note that the first series resistance *must* be able to sustain high peak voltage without breaking down. Common resistors are not suitable unless used in compensated arrays. In the past carbon composition resistors were typically used but suitable high voltage thin and thick film resistors are now available. Ceramic composition resistors may also be suitable.
To maintain AC performance, the series resistors are bypassed with capacitors but this lowers the protection rating as frequency is increased, leading to the common voltage derating versus frequency curve.
Protecting outputs is a more challenging exercise but it is very achievable. On a multimeter, the ohms mode has a current output which must be protected connected in parallel with the high impedance voltage input. Some old multimeters use an high voltage incandescent bulb as a fast response PTC thermister. A high voltage depletion mode MOSFET acting as a current limiter is also useful here. Or design the current source with very high voltage compliance which only has to go one way.