Sort of a tricky question.

There were certainly a lot of use of GULAG labor for the atomic program. The NKVD was in charge of it not only because of its expertise with espionage but also because of its expertise with mining, industrial production, and the like. So there's that, which it sounds like you know about.

In terms of actual research and engineering, it is a little trickier. Scientists were not "forced" to work for the atomic program. There were, however, some members who were in "gilded cages" — they were not fully free to leave, really. This is especially true of the Germans and Austrians who were picked up in the Soviet equivalent of the Alsos program, like Nikolaus Riehl. These people later recounted that their experience was somewhat halfway between having a job and being in a kind of prison; they were certainly treated better than GULAG laborers, to be sure, but leaving wasn't an option until a lot of time had passed.

I have not heard of anyone being assigned to research functions totally against their will, though. It is not really the style of the NKVD. Even in the sharashkas, the engineer/production GULAGs, the prisoners tended to be doing the work in a highly-motivated fashion, because the alternative was to be sentenced somewhere like the Kolyma, where cold and death awaited. Better to build bombers, if it came to that. But I don't think the nuclear weapons work was done in a sharashka-like environment; that's not the impression I get from what I've read of the Soviet program, anyway.

The best books on the early Soviet nuclear program are Holloway's Stalin and the Bomb, Gordin's Red Cloud at Dawn, and Brown's Plutopia.