In the English language, more generic terms are not capitalized, including many religious terms such as monotheist, polytheist, atheist, agnostic, schismatic, heretic, pagan, and so forth. Even though the English word gentile should fall into this category, most of the time when you see it employed these days it will be erroneously capitalized as Gentile, precisely because of this fallacious equivalence. However, it's become rare to even use this word, which has long been commonly replaced by the awkward term non-Jew, which is not even a proper word in itself, due to the demands of political correctness.
Since the Codex objects to political correctness and embraces tradition, it should prefer the term Gentile to non-Jew, and should also be willing to write it correctly as gentile while properly capitalizing the word Jew.