As many as 1.5 or 2 million people were imprisoned in the gulags at any one given time. While the gulags were known for being forced-labor camps, I believe that some elements of the system were desirable, and thus, should be retained, while rejecting the element of forced labor.
Another advantage of the gulag system – or, for that matter, any prison, or system of prison labor camps – is that it accomplishes the bare minimum of how to deal with dangerous criminals: take away their freedom in some way, while secluding them far away from anyone they might hurt, until it can be shown that they are no longer a danger to others, and have been rehabilitated.
In 2013, the U.S. incarceration rate was 716 per 100,000, and it peaked in 2008 around 1,000 per 100,000. Despite the mass incarceration of some 110,000 Japanese-American citizens during World War II, the incarceration rate for the U.S. never exceeded 140 per 100,000 during the war.
The Soviet Union, on the other hand - if we assume that no more than 1,500,000 were in gulags at any one time - can claim only a maximum 800 incarcerated per 100,000.
The U.S.S.R. under Stalin cannot boast lower incarceration rates than the U.S.; these rates were similar, and comparable; not wildly dissimilar in a way that should show favor of either power. The similar incarceration rates should not reflect negatively on either government any more than the other.
That being said, though, I think it would be fair to try to argue that, if two countries have similar incarceration rates, then if one treats its inmates more humanely, then that country would logically be the one with the less harsh prison system.
Edited and Expanded on January 4th, 2019
Edited on January 6th and February 14th, 2019
Post-Script Written and Added on January 6th, 2019