Gallinari's health worries me, and I think he's probably best defensively at the 4.
Tobias Harris is interesting, but there's the same problem there where he's probably best as a small-ball 4.
I'd prefer to wait until the Celts can acquire a wing who can reliably shoot the three and at least adequately defend the 2, 3, and smaller 4s.
In free agency next summer, Batum would probably be at the top of my list. Parsons could be an option as well, though his knee issues are concerning.
It's hard to find a good candidate via trade. Wilson Chandler, maybe? He's got health issues, too.
Finding a good 3-and-D wing in the draft might be the best way.