The Soviet system used psychiatry as a weapon by diagnosing political opponents as mentally ill in order to confine them as patients instead of trying them in court. Anyone who challenged the state such as dissidents, writers, would-be emigrants, religious believers, or human rights activists could be branded with fabricated disorders like sluggish schizophrenia. This turned normal political disagreement into supposed medical pathology and allowed the state to present dissent as insanity.
Once labeled in this way, people were placed in psychiatric hospitals where they could be held for long periods without legal protections. Harsh treatments were often used to break their resolve. The collaboration between state security organs and compliant psychiatrists created a system where political imprisonment was disguised as medical care, letting the Soviet regime suppress opposition while pretending it was addressing illness rather than silencing critics.



Link to source?
How about
“The History of the Gulag: From Collectivization to the Great Terror
Oleg V. Khlevniuk
Translated by Vadim A. Staklo”
https://www.jstor.org/stable/j.ctt5vkt98
Or
The Encyclopedia Britannica
https://www.britannica.com/place/Gulag
The book estimates 4,000,000 in gulag after the great purge in addition to another 2,000,000 corrective labor programs equaling 6,000,000 total, while the enyclopedia estimates 5,000,000 total. The 1939 USSR population historical data was 170,557,093.