FWIW, the symbol (two plates in a circle) is also sometimes used for normally-open relay contacts on ladder diagrams, or, I suppose, a capacitor that's special in some way (perhaps sealed in a vacuum, hence the circle emphasizing the envelope). There's usually a solid dot inside the circle, denoting a gas filling (gas discharge tubes (GDTs), neon lights, etc.). Alternatives to the straight lines include round, triangle/arrow, or 'D' shaped electrodes, emphasizing the nature or intent of the discharge (from neon glow discharge to high current spark gaps).
The "SG" designation and use definitely suggest at least an air type spark gap, in your case.
Tim