A gas turbine disk includes a rotor and a tie-bolt. The rotor includes a plurality of blades and a plurality of disks. The plurality of blades are disposed on outer circumferential surfaces of the plurality of disks. The tie-bolt is disposed along a center axis of the rotor and through a bore defined through hollow portions of the plurality of disks, so as to couple the plurality of disks to each other. A diameter of the bore is larger than a diameter of the tie-bolt. The plurality of disks respectively include a groove spaced from the bore in the circumferential direction of the bore, the groove being elongated in the axial direction of the bore such that cooling air flows through an internal space thereof.