Shale is usually more radioactive than clean sand or carbonate, because of potassium, uranium, and thorium in clay, so the curve is the standard shale indicator and a correlation curve between wells. The scale is API units.
A sand with feldspar, mica, or uranium can read as hot as a shale. Using the gamma ray alone will call that sand shale and drop it from net pay. A spectral gamma ray, which separates the three radioactive elements, is the check when the total gamma ray and the density-neutron disagree about whether the rock is clean.