It's not "na" but "nga", and I don't mean "n-ga" but "ng-a". Like the sound at the end of the English word "spring". It's an accent thing, some say it's personal preference but it's a little regional too. And you can't pronounce it as ng at the beginning of a word or after ん. watashi ga / watashi nga (both possible) - gohan (not ngohan) Some people only pronounce it as G, some only as NG (when possible), some will say both randomly.

I don't get this question because we never pronounce GA like NA. We pronounce HA as WA though