No need to retake the GMAT, unless you feel you can score like 760 or greater. But then again, even then I probably wouldn't bother. In your case, I'd focus on the written applications -- you're going into a post-MBA career that involves not just a shift in job function or industry, but in a way at least from an outsider who doesn't know you personally (adcoms) a shift in sensibility, values, and perspective.
You work in finance - in an industry that has a reputation for being culturally nihilistic, cynical and self-interested, and now you want to shift to another industry that culturally is progressive, ideological, and public service oriented. The key for you is to show that you have the soul of the latter (i.e. you are at heart the kind of person whose soul mates work in social enterprise; you just happen to work in finance right now). In short, you want to convince them that this sensibility is genuine, and not just "essay positioning/spin".
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