An item marked 1/20th 10K GF (gold filled) has a layer of gold over a base metal that equates to 1/20th of the weight of the entire item. This means that a brooch stamped 1/20th 10K GF and weighing 1 ounce, contains 1/20th of an ounce of 10K gold. Stated more simply, the overall metal of the item is about 2% pure gold).
The advantage of gold-filled, versus gold plated, is the thickness of the layer. A gold filled item that is well cared for can retain its gold surface for several lifetimes. Most gold-plated items will lose their gold finish within several years of consistent wear.