How empirically and historically accurate is The Gulag Archipelago, by Aleksandr Solzhenitsyn?
I have read the book. While I can't answer to specifics, since you haven't asked any, I clearly recall that, past 1991 or so, for about 4-5 years, there were recurring news items in Europe, where I was living at the time, about the discovery of mass graves from the Soviet era. Whether they have all been discovered, are not being looked for anymore or the news is now suppressed, there have been haven't been as many in recent years.
It is somewhat hard when searching to separate out WW2-related mass graves from political repression mass graves, but enough have been found that it would seem that the Gulag Archipelago's thesis of mass incarcerations and executions are not, on the whole, fully without substance.
Add to this the recognized policies that led to the Ukraine famine in 1932 - https://en.wikipedia.org/wiki/Holodomor - and it is hard to argue that the USSR could not be extremely coercive towards its citizens.
Is all of Gulag true and objective is another question - the book is full of carefully-stated examples of persecutions which were most likely obtained through first hand recollection and testimonies, so would be hard to verify from documentation that is probably missing or never existed in the first place.