A gulag was an infamous sort of prison in Russia established by the Soviet Union. They were known for extreme conditions, cruel treatment, and little hope of escape. Mulder and Alex Krycek were imprisoned in a gulag in Tunguska while investigating the origins of the black oil. They discovered that the Russian scientists were experimenting on prisoners by exposing them to the virus and then trying new vaccines on them. Mulder was subjected to this test but survived because the vaccine worked. Krycek was able to evade this because he was actually in league with them, for he sought revenge against the Syndicate and saw an ally in the Russians. Mulder escaped the gulag by commandeering a truck and driving away. (TXF: "Tunguska", "Terma")
The Tunguska gulag was still in use by the Russian conspirators in 1998 and was still home to Krycek. Krycek took prisoner a young boy named Dmitri from Kazakhstan after witnessing a massacre of abductees committed by the faceless aliens. Dmitri was subjected to tests with black oil following intense disfigurement of his face, which was found to be an effective deterrent against the oil. Krycek brought Dmitri to the United States as part of his revenge plot against the Syndicate. (TXF: "Patient X")