I don't think sticking with GTK for the OS X version is a wise idea. It really just needs to go Cocoa or not bother, since right now it's downright unpleasant to use for me. Scrolling through text using a trackpad, for example, feels completely unnatural next to the rest of the OS X UI (horizontal scrolling appears to barely work at all - vertical scrolling isn't too bad but there is something very off about it). I think the decision to go with GTK for the OS X version was just a poor choice and probably not one made by anyone who uses OS X often.
In the meantime, I guess I'll go look at the source and see what kind of changes would need to be made to use Cocoa in place of GTK on OS X.