Bill - why would you call it a G minor add 9? I would think that way that chord is voiced (Bb on top) would mean that the Bb is functioning as an extension, and not as the minor 3rd.
tonym has a good point -- it could also be called C7+9 or C7ALT, as there is no 5 in the chord. You could add b5, #5, and b9, and it would still be C7ALT, right?
This is cool stuff, but it's confusing at times.