The Gold Museum is located just next to the entrance of Sovereign Hill, in an ugly grey building. The ticket price of Sovereign Hill (a hefty 16-32AU$) includes entrance to the Gold Museum as well. You can visit the museum seperately for 4-8AU$, but I would only come here if you really want to visit Sovereign Hill, because the museum is reasonable only when it's free.
The museum is divided into a few parts. First part consists of a collection of different sized gold nuggets. Second part focuses on the history of the gold in the Ballarat areas, with pictures and postcards from the past, tools used like smelter pots or pickaxes, a quite interesting collection of Australian furniture and clothes from the time of the gold rush and historical books on these times.
The most beautiful part of the museum however, according to me, is the collection of gold objects from different cultures, like gold coins (many), religious ornaments (a few crosses and a chandelier) or ornamental plates (only a few).