Parks Victoria have a pretty useful website, and towards the bottom have details on walking maps etc.http://www.parkweb.vic.gov.au/1park_display.cfm
It's a spectacular region and there are lots of awesome walks, however if you're traveling solo, my suggestion would be to set up camp at one of the excellent campsites, and do a series of day walks, starting off at say Wonderland or Sundial.
If you do go ahead, the maps list (or at least my map does) instructions, tips etc for Overnight Expeditions, some of which are:
* Take a first aid kit, compass, sleeping bag and tent.
* For safety, walk with at least two others.
* In summer, most creeks in the Grampians are dry. In warm weather each person needs 2-4 litres of water per day.
* Plan your walk. A fit person covers 2-3 km per hour on a gently undulating track, less in steep terrain.