This chart shows typical turn-on performance of the 350 V, 2026 series GDT on a 10/1000 µs, 100 A surge. The GDT ionizes or conducts in less than 1 µsec after the transient voltage has reached the rated DC breakdown voltage.  The GDT rapidly (nanoseconds) reaches full conduction after ionization occurs. Please see the Bourns application note at www.bourns.com “First Principles of a Gas Discharge Tube (GDT) Primary Protector"  written by Tim Ardley, Bourns Senior Telecom FAE, for an in depth discussion of GDT operation.