The former main camp at Mauthausen became a camp for Displaced Persons who were waiting to emigrate to other countries after the war.
The Soviet Union also took charge of the quarry at Gusen and continued to take stone from it, operating under a company called Granitewerke Gusen. In 1955, the Allied occupation of Austria ended and the Soviets moved out; the railroad and the station at Gusen were dismantled. The property that remained after the Soviets left was given to the Austrian people. In the late 1950ies, the land in the Gusen area was privatized and private homes were built where the camps used to be.
It's a great and thorough explanation; you ought to take a look.