I’m no linguist or Japanese scholar, but it has long been interesting to me that the full word “gaikokujin” translates to “foreigner” (kanji for “outside,” “country,” “person”), while “gaijin” removes the “country” part, leaving something that could be read more fundamentally as simply “outsider”.

Nerds dramatically exaggerate the significance of this. Yes, if you just look at the kanji, "outsider" is possibly the more literal translation, but at the end of the day what matters is what is in people's heads when they use the word, and not the word itself.

"Gaijin" gets played up in weeby debates -- by people who are looking for reasons to be offended -- as some hugely significant cultural deviation, but it's honestly more like "foreigner" in practical usage. That word can be incredibly rude, or completely innocuous in different contexts. As can 外人. It's more accurate to say that both words mean "foreigner", but 外国人 is the PC version that renders you safe from criticism.

(The irony, of course, is that no matter where you go you'll hear plenty of borderline racist policy talk with the sweetest of language.)

this. I think outsider is both a more literal translation, as well as an accurate reflection of the attitude

Is it like gringo in Spanish or bule in Indonesian?

So maybe the Ninja Gaiden NES game should have been called Ninja Gaijin outside Japan?

If someone has only one Japanese parent are they Gaijin?

No, they are called Hafu. Half Japanese.

Do you know if that is a japanese adoption of "half" or is it based on pure Japanese etymology and just a coincidence it's sounds similar?

That one is a transliteration/adoption, as it is literally just ハ-フ (pronounced as "ha-fu"). The actual word for a half (like in "half a minute") would be 半 (pronounced as "han")

It's written using katakana, which denotes that it's a loanword.

my wife is half japanese,has citizenship and has been called a gaijin so yes. the guy was being an asshole so everyone doesn’t necessarily feel that way. but they kinda do