Gold filled (also gold plated and gold shell) refers to a case made of steel or another base metal that is covered with gold.
Most gold filled cases are electroplated with 18 karat yellow gold. The layer of gold on such a watch varies, but is typically 10 to 20 microns but some are as thick as 100 microns. The term has caused confusion for consumers, who often assume it refers to solid gold.
In contrast a gold shell case is covered by a thicker layer of gold pressed to shape and glued or soldered to the base metal case. Rolex gold shell from the 1950s was 240 microns thick and only covered the top and sides of the case. Gold shell is easily visible as a layer of gold when viewed from the underside of the case.