Maybe a Christianese word, most of the times used in place of the more literal word "nations".
While I agree it's a bit Christianese, I'm not sure of a word or phrase to replace it with. The trouble with just "nations" is that it more properly means "non Jewish nations" or "non-Jewish people", which is a mouthful, and is what "gentile" means. Again, "non-Jewish nations" or "non-Jewish people" would work in many places, but is a mouthful. Do you have a more concise alternate suggestion?
The Greek word in isolation means "nations" and in many places is translated as just "nations" (and not "people", which is a different word). Who is to say when it is used to refer to non-Jewish ones? Possibly a footnote, but not the text itself (at least not in a word-for-words translation). For example, the word for "Greek(s)" is left untouched, no matter that in many instances it really means "non-Jew(s)" and thus "gentile(s)". The same way that keeping "Greek(s)" in these instances may be useful for the reader to know that (Koine) Greek was the international language of the time (and the language of the New Testament), keeping "nations" may also be useful to know that Jews had incorporated their chauvinism in the meanings of some words.
You make some good points. I'll add it to the list of words I keep to go back through and double check after I'm done my editing pass of the currently translated text. I'd want to look at all the places it's used at once to get a better sense of the word.