"At the peak of the GULag system, shortly after the Second World War in 1951, 2.4% of the adult Soviet population was incarcerated, which represented 2.5 million prisoners. For comparison, 2.8% of the United States population is currently incarcerated, millions more than were ever imprisoned in Soviet gulags. "

We need to do the math ourselves on these numbers, both absolute and relative, and for both countries.

First there's a semantic problem. Do the percentage figures (2.4% for the USSR and 2.8% for the US) represent people strictly in prisons, or just "in the system"? The USSR and US system are completely different and it only makes sense to me to compare people strictly behind bars and not "in the system". USSR had health institutions, whole villages, etc. under the gulag administration, and US has bail, parole, house arrest, jail (awaiting trial) etc.

Second, I just can't confirm the numbers myself, both for relative and absolute terms (percentage and absolute numbers). So the burden of confirming those numbers actually falls on us, and we have to figure them out once and for all. And also source them, because only one number is sourced and that's the 2.5 million figure (source is misattributed currently, it's reference 1 found in the article after the US number).

We are also comparing the adult USSR population against the whole of the US population in the article.

So the first task is to confirm the numbers ourselves in a way that's actually coherent, and second I would suggest to move this part into its own section, and there we can go over why people in the US are imprisoned (many non-violent charges such as drug possession) and compare that to the prisoners in the USSR, since we know from the opened archives (and the article says it as well) that most of the prisoners were actual criminals. This would also make the introduction shorter, it's too long currently.