Imo, there is a difference between ideals and practice here. Sure, socialism as an ideal is great. But the ways that Lenin and Stalin implemented it were not. In fact, I would argue that neither of their regimes were socialist, rather totalitarianism dressed up, particularly under Stalin.
Although you are right that 'gulag' is not technically a word, it's a Russian acronym, and has come into common usage.
define: gulag - Google Search
http://gulaghistory.org/nps/
http://gulaghistory.org/nps/onlineexhibit/stalin/
http://en.wikipedia.org/wiki/Gulag
Plus, you can't deny the terror of the Leninist/Stalinist regime purely on the basis that you don't think *one* of the terms used during this time is incorrect. Even if the gulags did not exist, there would still be the question of the devastating effects of the Civil War, War Communism, collectivisation, dekulakisation, the Great Purges, the NKVD, the Cheka, etc.