"Gaijin" is any non-ethnically-japanese person, so non-japanese long-term residents are still gaijin, even if they've been here for 15y, have a long-term job and married a japanese.

"Local gaijin" are thus simply people living in the area but not of japanese ethnicity. Mostly expats.

I understood it as referring to non-Japanese people living in Japan, who should make sure that the clueless ones among their ranks behave.

That's exactly it. I try and help foreigners who look lost at stations when I can, and while I'm a foreigner myself, I have been here 8 years now.

Aka expat community. Those who have already moved there should work to try to help newcomers integrate.

Aah, I guess they're still considered gaijins.

You're considered gaijin even if you're born in the country. My half-Japanese children (born and bred in Japan) get called "gaijin" all the time.