It looks like you need to add just one more stop—a dark one—to complete the lower half. Place it very, very close to the light stop (so close that they're almost on the same point).
I (could swear I had) tried that but was not getting the effect I was looking for, the lighter area was still gradating in both directions. I just did it again following your reply and suggestion and it looks great. Maybe I did not have that additional, darker stop close enough.
Thanks so much. I was about to... well nevermind that.