After the invasion of North-East Poland (now Belarus) He was interned at a work camp in Uksora, Siberia. left Russia via Persia and joined the Poles who would eventually fight at Monte Casino. He died some years ago, he mentions this in some written notes he made during the 1970s, as well as actually detailing the conditions in Russia. Note: I may be mistaken about the NKGB, he states the KGB but I'm sure that was a later development. Many thanks to those who can help!
I can't speak with confidence as to the exact paperwork to be completed by the departing Poles but it sounds perfectly plausible. There would have been some paperwork - the NKVD could be sticklers for procedure when they wanted to be. We know, for example, that they recorded the release of some 119,855 Poles in Anders' army. The figure may be wrong but at least it's exact.
In the same vein, typically released Gulag inmates were issued with a certificate of release and a certificate of rehabilitation or withdrawal of sentence. I've seen accounts, from the post-Stalin period, of returnees being given warnings, written or informal, to keep silence about what they'd been through but I can't say to what degree this was standard. I'd also be slightly surprised that the NKVD would bother with a non-disclosure agreement for Polish prisoners.
So your grandfather would have signed something and it could have been a silence agreement. If he didn't speak/read Russian then I'd suggest that it's more likely to have been a standard release form but I've got no real basis to challenge his story. Someone else might be able to comment specifically on the Polish releases in '41.
Note: I may be mistaken about the NKGB, he states the KGB but I'm sure that was a later development.
Probably not the NKGB but this is a matter of semantics. The NKGB was the former the GUGB, the 'State Security' department of the NKVD. (The NKVD itself was responsible for a whole range of internal policing functions.) From Feb-Jul 1941 the GUGB was given an independent role as the NKGB before being folded back into the NKVD when the war started. After the war it would again be split off as the MGB before becoming the KGB after Stalin's death.
(If that isn't enough acronyms, the NVKD itself had started life as the Cheka, before becoming the OGPU, NVKD and finally the MVD. Departments and responsibilities often shifted in these frequent reshuffles.)
So when your grandfather was released (presumably into General Anders' army) it would have been by the NKVD in post-August 1941. But, as I say, that's largely semantics.
[Edit: Forgot sources. Polish release numbers come from Wheatcroft's Scale and Nature of German and Soviet Repression and Mass Killings and Adler's Gulag Survivor: Beyond the Soviet System has some details on release certificates/processes.]