"It is easy to misjudge the Gulag’s contribution because its more lasting monuments—the Moscow metro, the Moscow University, and the Norilsk Nickel Combinat—are what remain. Forgotten are the “roads to nowhere,” long fallen into the decay that is not unique to Gulag projects. The countries of the former Soviet Union are
cemeteries of failed construction projects, which would never have been started if project analysis had not been distorted by the absence of market pricing in the national economy and by the country’s isolation from international markets. Many such projects came into being merely because of fleeting political considerations." [1]
"It is easy to misjudge the Gulag’s contribution because its more lasting monuments—the Moscow metro, the Moscow University, and the Norilsk Nickel Combinat—are what remain. Forgotten are the “roads to nowhere,” long fallen into the decay that is not unique to Gulag projects. The countries of the former Soviet Union are cemeteries of failed construction projects, which would never have been started if project analysis had not been distorted by the absence of market pricing in the national economy and by the country’s isolation from international markets. Many such projects came into being merely because of fleeting political considerations." [1]
[1] http://www.hoover.org/sites/default/files/uploads/documents/...
Typing "moscow metro forced labor" into Google returned the PDF cited above as the second entry in the search results that were returned.
Perhaps you could provide a source for your claim that there is no connection between the construction of the Moscow Metro and the Gulag system?