Much of the "progress" in the USSR was based on slave labour [1]. And it was not limited to just physical labour. Many intellectual feats were also the product of people working in, what essentially amounts to, a prison [2].
According to this study [0] the gulag deaths were approximately 830,000 from 1934 to 1953. It is important to know however that 70% of the deaths occurred between 1941 and 1944 (included) so they can kinda be attributed to difficulties from War Period. Also, it's important to note that antibiotics didn't become available until after WW2.
To put things into perspective, I have an interesting comparison for you. Using the same source as above for the USSR, and this report [1] from the Bureau of Justice Statistics we can say that Mortality in the gulag in 1953 (236 deaths per 100,000 prisoners) was lower than mortality in US prisons today, both in state prisons (303 deaths per 100,000 prisoners) and federal prisons (252 deaths per 100,000 prisoners).
Hope it's useful.
[0]: https://www.jstor.org/stable/2166597?read-now=1&refreqid=exc...
- Cancer (1,128)
- Heart Disease (1,025)
- All Other Illnesses (525)
- Liver Disease (260)
Healthcare in prisons is terrible: https://www.propublica.org/article/the-prison-was-built-to-h...
Taken as categories, 44% of people in prison die as a result of illness, 31% as a result of suicide. (both sourced from BJS still) Both avoidable with the right funding, and yes, atrocities. If we're going to have a justice system that lives up to what it claims, a side effect of a prison sentence shouldn't be death.