Gtst33 Backfire

[bluey33]

Gday guys, car has just started to backfire a little after i accelerate or just as i hit the gas around 2000rpm... Not everytime but enough to frustrate me. Could be many things that im aware of coil packs, AFM, plugs. Just wondering what spark plugs to use?? Ive seen different gaps for the plugs being used aswel... Just want an answer for plugs and gap?

The car is series 1 gtst33 stock with cat-back.

Also is it bad for the car?

Thanks heaps, Bluey