I think it was a good thing, so long as the class dynamic is a correct one.

If used as a tool to oppress the proletariat, then it would be a bad thing. If used as a tool to repress and reeducate the bourgeoisie and petite-bourgeoisie, then it would be a good thing. Gulag is not a death camp, but a manifestation of the Dictatorship of the Proletariat. When the old oppressing classes attempt to overthrow proletarian society and replace their oppression, they themselves will be oppressed and proletarianized. Not forever, but for a limited duration, while the remnants of the dictatorship of the bourgeoisie are being broken and scattered. Nor would they have to work beyond their own means of subsistence. They must earn their bread, like any other proletarian, but I don't think they need be exploited for surplus-labor.

You can argue whether or not the historical Gulag system measured up to this standard, but I don't think it was wrong in principle. No, the petite bourgeoisie and bourgeoisie will not like the Dictatorship of the Proletariat, but they don't get a say, just like the proletariat does not get a say in the Dictatorship of the Bourgeoisie.

Also, I would disagree with the statement that the DotP was abandoned by Stalin. Yes, there were still classes under Stalin, but between 1924 and 1953 the old classes were routinely repressed, socialist labor became the dominant form of production, and the Workers and Peasant classes were the classes from which the Communist Party and intelligentsia were drawn from. The class character of society was proletarian, even if bourgeois leftovers were still crusting in the dark corners of society.