as in the bleach Bankai well the ai being pronounced like eye
This is mainly because of a dyslexic friend who ran planescape and said it that way and the fact that the human brain generally only looks at the first and last letter of a word, I'd never really examined the word properly before.
But looking at again it could still be pronounced Gen-a-si with the a being near silent.
Someone ask Monte Cook how they're pronounced, since he created them in the Planewalkers Handbook. That'll be the answer there if we go with original intent.
Unfortunately I don't think genasi are mentioned in the audio portions of the Players Primer to the Outlands, which has always served as a handy planar pronunciation guide on a lot of terms.