Are you sure it has a spark gap? The last few small portable plasma units I've worked on were inverter designs and didn't have spark gaps.
It should be easy to see, and likely close to your HF transformer. If you don't have an idea of what to look for, you could open the case and power the thing up with compressed air attached; when you trigger it you will immediately notice the small arc across the gap. Set the gap with a feeler gauge to spec. usually by backing off a lock nut and turning a screw.