To test a GDT with a multimeter do the following

 

• Set your multimeter to the resistance (ohms) or continuity mode.
• Place the probes across the GDT terminals.
• A healthy GDT should read OL (open line) or extreme megaohms of resistance.

 

Testing the actual breakdown voltage requires a variable high-voltage DC power supply with a strict current-limiting resistor (such as 100kΩ) or a specialized GDT/insulation tester.