Intrinsic factor (IF), also known as gastric intrinsic factor (GIF), is a glycoprotein produced by the parietal cells of the stomach.It is necessary for the absorption of vitamin B12 (cobalamin) later on in the small intestine. In humans, the gastric intrinsic factor protein is encoded by the GIF gene.t is necessary for the absorption of vitamin B12 (cobalamin) later on in the small intestine. In humans, the gastric intrinsic factor protein is encoded by the GIF gene.