Except that US prisons do not force inmates to preform labor that have a net benefit to the state -- inmates across the US have a net cost to taxpayers, and for the most part, people are not tried/jailed for political reasons. The prison system also does not actively kill people, except those who are sentenced to death, and takes active steps to protect inmates from being killed by others.

Gulags on the other hand, were essentially forced labor that provided a benefit to the state, people were often there for political reasons, and many were actively killed by the state. Some people believe the majority of people in Gulags were killed.