Spark gaps (and a spark plasma) are extremely effective surge suppressors, much more so than any semiconductor device. The reason is, once the spark plasma formed, in air or a gas discharge tube, it has a negative resistance, as the current increases the geometry of the plasma grows thicker. So what happens is, it simply assumes a constant voltage across it terminals, no matter how high the current (within limits of course until the total energy explodes the chamber or melts the metal). Thin looking sparks are low current sparks, fat ones are higher current sparks. (one definition of a spark plasma is something that has the physical properties of a gas and the electrical properties of a metal)
A spark in air for example over a 4mm gap, has a fixed voltage drop around about 500V once formed. In a car's combustion chamber, it is a little higher at around 1000v. When people get a plain wire and use it to discharge a CRT anode connection, the voltage there typically for a small VDU is in the vicinity of 10kV, as the wire approaches the anode connection, before it gets there, within about 1/2 to 1/4 an inch, the air ionizes and the spark plasma is formed between the wire end and the anode connection. The spark there in that gap adopting a near fixed voltage across its length, due to the negative resistance effect. So the capacitance of the CRT bulb, typically in the order 500 to 1000pF is effectively shorted out and the peak discharge current into the low Ohmic wire conductor is briefly many thousands of amps, limited a little, by the inductance of the wire. It is never a good idea to directly short out charged capacitors. It is much better to use current limiting resistors. If the capacitor is charged to a very high voltage initially, a very high Ohmic value resistor is required for a gentle discharge. The initial discharge current is simply calculated from the capacitor's terminal voltage, divided by the resistor value. It starts at that value briefly, then falls in an inverted exponential manner over time.
Very large gas discharge tubes are used in Aircraft & rocket exciter units as part of a spark generating system, I have attached a photo of some of the ones in my collection, some contain Krypton 85.
