GDTs are very sensitive to dv/dt changes. A slow transient (e.g. 100 V/s) will cause little “overshoot”. In this case, the graphs shown here are for a 230 V GDT. This means that its “DC Spark-over Voltage” is 230 V: when a 100 V/s ramp is used, the device will activate at about 230 V (typically +/- 20%). However, when this same device is subjected to a faster surge with a ramp of 100 V/µs, the device will only activate around 500 V. For an even faster “impulse spark-over” of 1000 V/µs, the same device will only activate around 600 V.