Clothing/Shoes/Weather Gear: If you are headed to the Grampians during the September timeframe, definitely remember to bring a windbreaker or light jacket. It was not cold, but the breeze was blowing! September is the crossover period from winter to spring here, but it is better to be safe than sorry!
Good shoes for walking in The Grampians are essential, especially if you want to take some of the longer walks. You'd find some of the rocks a bit sandy/slippery to climb without proper shoes.
Toiletries and Medical Supplies: As Halls Gap has only a handful of shops, you'd be best to buy most things before you come, as they'll be more expensive here, and there's no chemist, just a supermarket and other shops.
Photo Equipment: You should be able to buy normal film here, but it will be more expensive so I'd recommend you purchase it beforehand at a department store.
Camping/Beach/Outdoor Gear: This is really a great place to camp, so bring your gear if you have it.