Capitalism is the problem for the working class that motivates unions as a mitigation.
Since prisoners do often work, you could technically call a prison a publically-owned workplace provided it is actually owned by the public. It would be highly misleading, but technically not entirely wrong. But gulags weren't owned by the public but by an unambiguously dictatorial state, so no, they aren't.
I hope this helped clear things up.
 Calling gulags prisons is quite an understatement, but very bad prisons aren't worker's cooperatives either.
Its terribly inefficient when compared against idyllic, abstract notions of societies, where resources are distributed and used with perfect efficiency. But so is every other practical alternative.