I think the Soviet Union did contribute greatly to the Allied victory in WWII. Hitler put a great deal of his power into invading the Soviet Union, and, although the Russians suffered outragously high loses, the did utterly devestate the German army in two major consequetive battles, Stalingrad and Kursk, not to mention tying up a ton of men and equipment in the 900 day siege on Leningrad.

Would the allies have defeated the Nazis without the help of the Russians, probably, but the war would have been much longer, and much more costly to both the UK and the US, especially if Hitler and Stalin had maintained their earlier alliance.

This is not to say Stalin wasn't a tyrant and a monster, he was. Your figure of 20 million people killed in the Gulag system is actually a pretty conservative one. However I do feel the Russians made a vast contribution to the Allied victory of WWII.