Is there no way to display an o with a macron (ō) in the Gill Sans supplied with OSX / iOS?
I just checked the font, and it's correct, that character is not there in Gill Sans, Gill Sans Bold, Gill Sans Italic, or Gill Sans Light Italic.  In fact, I've been checking out Monotype who made the font files, and they don't have that character in most of their versions of the typeface either.  Monotype does offer a Gill Sans Cyrillic Volume which you could license, if Gill Sans and the o with a macron are both essential to your app.  

From Monotype's site, here's a sample of Gill Sans Cyrillic Regular: