This same situation is true for us little people in the real world.
Common assault: hitting someone with superficial injuries.
Aggravated assault: kick or use a weapon (like knuckle dusters).
Causing injury intentionally: kind of obvious. Breaking kneecaps for the vig, etc.
Causing injury recklessly: leaving your car in drive and it rolls back smashing some guy or other negligent act.
Grievous bodily harm: Intentionally causing injury with something more than a single hit. Like setting a trap.
Take this off the football field and it seems likely that Gaff would be looking at aggravated assault (up to 1 year in prison) or maybe causing injury recklessly (up to 15 years), but that second seems extreme for a single punch.
So the most relevant question seems to be is an assault taking place on a sporting field not an assault?
(for Vic. I imagine that WA is similar)
https://www.victoria.criminallegal.com.au/crimes/assault-offences-vic/