How did the Gulag system belp Lenin and Stalin gain complete control over the citizens of the USSR?

It kept commentators and objectors away from the  population by captivating them in isolated  labor camps. Normally, they were executed if they showed to be a vast threat

Their purpose was to imprison political opponents. It was like a working camp for prisoners where they were treated like slaves. Whoever was a political opponent would get shipped to one of the gulags in Siberia and work hard in mines or things like that probably until their death.

the answer to your question is:

strong-mayor system

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