Kamiya just shot himself in the foot.
I don't know how much the rest of you know about Japanese culture (I'm an expert), but politeness and social etiquette are huge parts of it. It's not like it is in America where you can become successful even if you're racist. If you behave rudely in Japan, you bring shame to yourself, and the only way to get rid of that shame is repentance.
What this means is everyone who's not Japanese, after hearing about this, is not going to want to purchase anything Kamiya has worked on, nor will they purchase any of Platinum's future games. This is HUGE. You can laugh all you want, but Kamiya has alienated an entire market with this move.
Kamiya, publicly apologize for saying "gaijin" on Twitter or you can kiss your career goodbye.