The Gas discharge surge protector consists of a glass or ceramic tube with an electrode in each end. The tube contains a low pressure gas mix. When the voltage over the tube rises over a certain level, the gas starts to ionize. When the voltage rises further(to the so called arc voltage), it causes an avalance effect. This means that fast moving electrons from the ionized gas collides with other the other gas atoms and causes further ionization. The ionized gas is a good conductor, and the resistance can fall as low as 5 mili ohms. The switching speed of the gas discharge tube can be down to 1 micro second. By using different gas mixes the arc voltage can be changed.

This kind of tube is used to protect against surges caused by lightning, switching of inductive load , EMPs and electro static discharges. To protect against pulses with nano second rise times, it is used in parallel with a voltage dependant resistor. This circuit has the advantages of the VDR's fast switching time(<25nS) and the discharge tube's abillity to conduct high currents(>10KA).
An example of a circuit with discharge tubes and VDRs
(For 230V AC)

~   --------*---------*---------------.
            |         |  |            | 
           /V\       | |/  VDR 1      |
      SP1 |   |      |/|   150V       |
           \^/      /| |              |
            |         |              | |
GND --------*---------*------------| | | LOAD
            |         |  |           | |
           /V\       | |/  VDR 2      |
      SP2 |   |      |/|   150V       |
           \^/      /| |              |
            |         |               |
~   --------*----------*--------------'