Were Gulags still a thing during World War 2?

The Gulag system consisted of labor camps for prisoners and those sent into exile by Stalin which were designed to exploit the Soviet Union's vast resources of minerals and lumber (the worst camps were logging camps, not the famed salt mines of the Kolyma). They continue to be "a thing" today.

Do you mean, did the Gulags still exist during WW2? Yes, they did.

Yes, but the population in them decreased, because the mortality rate increased.

that was the high point of their use

The gulags existed well into the 1960s.

Oh yes, and for years afterwards.

Oh yeah, Stalin killed or imprisoned ANYBODY who disagreed with him. The gulags were crammed to the brim during his reign (with both foreigners and his own people).