I think there is one point we can all agree about. There are no reliable data on statistics of prisoners in Gulag. This is recognized by all scholars, including Applebaum. The Zemskov's data are just as good or bad as data described in the book by Antonov-Ovseenko. Yes, his book is a reliable secondary source per WP:RS, just as book by Applebaum or a book (if any) by Zemskov.    