What guage brads for a plywood ceiling

That depends on how thick the ply is, and what you are nailing it to.

Regards Bradford

Good point.
12mm ply onto 75x36mm treated pine fixed on the 76mm flat onto the same size ceiling joists.
My original ceiling was going to be gyprock and the cornice would have covered the gap between the wall plate and the joist but now that I have decided to use ply I will run some left over timber at right angle to the existing joists on the flat which will drop the ply ceiling 12mm below the brick work and finish of with some quad or cove.
What spacing for the nails and how many per 1200 wide sheet.