A rotor disk assembly for a gas turbine maintains a sealing capability even though adjacent labyrinth arms are dislocated from each other due to torsion or similar relative movement by thermal expansion or by rotation of the rotor disks of the gas turbine. The rotor disk assembly includes a plurality of rotor disks axially assembled to each other, the plurality of rotor disks including adjacent rotor disks coupled to each other by Hirth parts. Each rotor disk includes two labyrinth arms that extend axially and bilaterally and are located on the rotor disk more radially outward than the Hirth parts, and a first labyrinth arm of the two labyrinth arms having an end surface in which a receiving groove for receiving a seal ring is formed.