Stepan Grigorich never actually appears in the book, but we do hear some about him from Shukhov and his fake medical orderly, Vdovushkin. Grigorich is either a really mean, or a really stupid, doctor. Shukhov doesn't care much for him.
He was fast and furious, always on the boil himself, and he made sure the patients got no peace. [....] Work, he reckoned, was the best medicine of all.
Work is what horses die of. Everybody should know that. If he ever had to bust a gut bricklaying, he'd soon quiet down. (48-9).
Grigorich demonstrates the problems with the people in charge of the camp. For one thing, they are totally out of touch with reality; they force the zeks to do things that they'd never do themselves. And these things often demonstrate a lack of common sense – working all the zeks to death isn't really the best long-term management strategy. And given how the guards spaz out when someone goes missing, the powers of the camp probably want to keep most of their prisoners (relatively) alive and functioning.
Grigorich also represents the sort of outright mean people who run the camps. Making sick people get up and work is just cruel really. And by the end of the day Shukhov, who had been feeling ill, decides that he doesn't want to be "doctored into his grave" (835) by Grigorich and his minions, some of whom lack medical licenses.