Endocat: gall soap is a "traditional" soap. It's sold like any other regular soap in bars and cubes (though I've seen the liquid version of the soap as well), except that it's usually light gray or dark yellowish in color. The difference between gall and regular soap, is that gall soap doesn't make suds (or very few suds) when making contact with water.
Traditionally gall soap is used for cleaning stuff, usually for getting rid of stains from all sorts of fabrics. Sometimes people used to wash hands, because it doesn't have that many chemicals in it, and doesn't cause skin irritation (plus it has a nice smell).