When ignoring the period of 1941-1944 (nazi occupation of the soviet union and ww2) where 70% of all deaths in gulags occurred, the program actually had an incredibly low death rate for its time. By 1953 the gulag system had a lower death rate than current modern day US prisons have. Fact.
According to this study the gulag deaths were approximately 830,000 from 1934 to 1953. As said above however, it is important to know that 70% of all these deaths occurred between 1941 and 1944 (included) so they can be attributed to difficulties from the War Period and nazi occupation. Also, it's important to note that antibiotics didn't become available until after WW2, this contributes significantly to earlier higher death figures.
To put things into perspective. Using the same source as above for the USSR, and this report 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). Also the deathrate is likely much much higher than this report right now because of covid.
US prisons kill more currently than the Soviet prisons(known as the gulag system) did post ww2.