Gallo is a power forward now, basically. If he came to the Celts, I'd expect him to start at PF in a lineup with IT, Bradley/Smart, Crowder, and either Olynyk or Johnson.
Ideally, you would try to also acquire a better defensive big man / interior finisher (e.g. Noel) to put next to him.
Gallo does take a lot of shots from outside, but he does have the handle to iso against slower players, and even be the ballhandler in a pick and roll occasionally. He's a skilled player, and he does drive sometimes. He's not a wing version of Ryan Anderson, in other words.