Does anyone know where to find the minimum spacing of grid on netting protecting a luminaire in a gymnasium in the code book?

You did not say what type of fixture, the only rule that I know of is 26-608 which refers to arcs and globes this says 32mm. Hope this helps.

There is no "minimum" requirement in either the 19th or the 20th edition of the CEC.

Rule 26-608 would apply only if the device in question is an "arc' lamp.

Rule 30-316 states that "lamps shall be guarded". So if the guard is adequate to the task, the CEC requirement is met.

You would need the Standard for wire guards and is not covered in part 1 of the CEC. A manufacturer of wire guards might have a copy of the standard.