I'm not saying the word should be rejected, simply that, as Gulag is more common than gulag (the evidence for this is on Google Ngram Viewer) its status should be at best 'uncommon'.
It would appear that the word gulag has, for some users, become a countable noun meaning 'forced labour camp' in which case it would be written in lower-case; I'm still not convinced, however, that the word should be common, especially as most of these horrific places have, hopefully, ceased to exist,
If
gulag was universally, or near universally capitalized, perhaps it wouldn't even be considered for inclusion in the Chihuahua lexicon. Although the Google ngram viewer shows that it is more frequently capitalized, the large proportion of uncapitalized occurrences at least indicates the propriety of inclusion. The Google ngram viewer is not the whole story, of course, since it only covers books. I previously cited my search of the available online
Washington Post archives, which showed an overwhelming occurrence of the uncapitalized version. Searches of the Corpus of Contemporary American English had similar results, with 711 hits. The British National Corpus, available at Brigham Young University, shows an overwhelming slant toward the capitalized version, with 27 hits. The Corpus of Canadian English (Strathy), also available at Brigham Young University, shows a much more even mix, with gulag/Gulag occurring 26 times.
It is no wonder that the Wikipedia article Mike referred to has universal capitalization, since it deals specifically with the historical Soviet forced labor camp system. The dictionary links provided on the Chihuahua main page, including Wiktionary, plus one that used to be there but no longer is (thefreedictionary.com), all show the uncapitalized version, with some also indicating that it can be capitalized. They all point to the increasing use of
gulag as a concept rather than a historical entity.
To me, the question of whether
gulag should be classified as common or less common hinges on whether or not reasonably well read speakers of English would know the word.