@moozooh
English dictionaries don't include Japanese honourifics either, but many translators choose to keep them anyway, because there is no translated way to maintain the meaning they're meant to convey. The question is not whether or not the word/meaning is in the dictionary, but what the best way is to convey the original meaning. While sure such usage of "gap" has not become wide enough to be recognised by the general pupblic, as long as it's recognised enough in this anime/manga community, it could be used to properly get the meaning across. I see no issue to the same exact communities using the terms that best describe their concepts, just because they're not standard English. We're not talking about standard English concepts in the first place, so why limit ourselves that way?
Also concerning your own examples, while "contrast" would have been more or less good, I think "quirk" doesn't quite describe it very well. The concept of "gap" as used by the Japanese is about the contrast between one and oneself, while "quirk" is about how one is different from others. Doesn't really work.