The difference between the GULAGs in the USSR and NK and any Nazi work camps for Slavs was that neither the Soviet nor NK camps had the goal of killing the workers in the process of getting work out of them. Those laborers who arrived with specialized skills (like machinists or electricians) or who somehow acquired those skills would be maintained as valuable slaves, although in miserable conditions and probably sterilized as a matter of course "to protect Aryan women from contamination". At least in the USSR zeks had a sentence, so if it was 10 years they would be released after 10 years, and so forth (at least for most of them). ALL of the Untermenschen not immediately sent to a death camp or starved out of hand were, in effect, chattel slaves with no hope of manumission. Their children, if allowed any, would be born slaves.
While the Nazis planned for vast killings, they also planned to get labor from many before death, and for an indefinite period survivors to be slaves. Over time you would see either a stable situation with slaves for labor, medical experiments, who knows what else would be maintained (like slaves in other slave societies) or if and when the need for slave labor went away they would die out. I very much doubt the Nazis would be killing Untermenschen and putting themselves in a labor shortage because of it. Obviously as agriculture mechanizes, you need fewer and fewer farm workers, but the wealthy and the middle class will want servants, there is always low level labor better done by slaves than Aryans, etc.