Put to hard labor and short rations, often in places like Siberia, quartered in inadequate shelter, given little to no medical support, supervised by people that didn’t care much if the prisoners lived or died - and often would prefer that they died - and worked to death.
It was hard, brutal and bluntly inhumane. But it wasn’t much worse than what the Germans themselves did to the Russians and others, both military and civilian, in their push east. Ivan, it turns out, has a surprisingly long memory and is a very good hater if you exceed his rules of conduct; the surrendered Germans found out *all* about that the hard way.
about half of the Stalingrad prisoners never made it to the gulags. They died on the march to the prisons.