What is gradient gel?

Some articles on gradient gel, gel, gradient:

SDS-PAGE - SDS Gradient Gel Electrophoresis of Proteins
... SDS-protein particles do not migrate freely at the border between the Cl¯ of the gel buffer and the Gly¯ of the cathode buffer ... The boundary moves through a pore gradient and the protein stack gradually disperses due to a frictional resistance increase of the gel matrix ... Stacking and unstacking occurs continuously in the gradient gel, for every protein at a different position ...