From that patent, using the stepped electrode shape :
"... 0.0005" gap, a DC breakdown of about 400 volts and an impulse breakdown of about 460 volts have been obtained.
For a 0.001" gap, breakdown voltages of about 550 and 650 respectively have been obtained.
For a conventional gap of 0.002", the impulse breakdown voltage can be from 1200 volts to 2500 volts, or higher"
At that tiny a gap, no way you can make glass that precise, heed a spacer. I think it's easier to use a larger gap and radioactive elements.
I worked on lightning protection, tested at
Hydro-Québec testing laboratories, for the pipeline industry. They usually test electric utility stuff like breakers and transformers, bushings etc. to impulse voltage of 5.4 MV, a switching impulse of 2.7 MV and a peak test voltage of 1,800 kV AC or 1,200 kV DC.
GDT's are unfortunately slow, the transient pass-through before they conduct still damages things.
It's strange to see them used in some multimeter front-ends.