Hello radiola,
You are basically asking: how to create a spark gap device?
The first thing you do is read the tutorial on how to create custom devices:
http://www.ni.com/tutorial/3173/en/
You should be able to get the correct SPICE code from the manufacturer. Where did you "heard" that recommendation?
What is the particular issue? One component missing? Also if you want to make a suggestion to improve NI Multisim you should post your concerns in the idea exchange forum:
http://forums.ni.com/t5/Multisim-and-Ultiboard-Idea/idb-p/multisimideas
Do you have any doubt about one of the steps of the custom components creation process?
This particular issue was discussed on a old forum in which another person shared his model and his ideas:
http://forums.ni.com/t5/Multisim-and-Ultiboard/Sparkgap-model-Spice-ABM-circuit-netlist-error-messag...
This forum was answered by Tien P, who has a lot of experiece in NI Multisim.
Randy @Rscd27@