It's hit and miss, and really hard to predict.
The coast is packed for the race, and obviously transport around downtown surfers gets a little iffy because of it. Coomera should be quiet and provided you're not staying near the track, you shouldn't have much problem - as for attendance, the race format has two 300km races (there was some talk last year of making it a single 500km race on sunday but it wasn't a popular idea) so the race fans are probably going to watch both days if they aren't local.
For my money i'd suggest it'd be a pretty normal October weekend attendance - you won't notice much up or down on regular numbers - you might lose a few petrolhead locals, but you also might get the 'rest of the family not interested in motorsport' going to a park to replace them...
If you're concerned about the crowds, and you don't get to visit often, but want to make sure you ride everything - budget for a RideExpress pass, but don't pre-book it. If you get to the park and its busier than you thought, go and get the Ride Express pass.