I live 25 min from Halls Gap
There are some areas (or at least there used to be) where you can walk in camp, but I would definitely be checking with the Rangers as there is still a lot of damage from the floods earlier this year.
Overnight Walking informationhttp://www.brambuk.com.au/assets/pdf/gr ... alking.pdf
Maps with the roads and walking trails are available (one for South and one for North sections)http://parkweb.vic.gov.au/explore/parks ... ional-park
Parks Victoria on 13 1963
Hope this helps or if you have more specific question just ask.
There are plenty of other great parks / state forests out this way
if you can't find what you are looking for in the Grampians.