On the brighter side, Gallo provides good contrast to G Will as the backup PF. Allows BOS to go with a more offensive minded option when G Will's defense is not called for. This will offer useful lineup flexibility.
I expect diminished returns from G Will / Gallo as they are more competitors for playing time than able to work together. Some together but better apart.
Some of the value will also depend on Udoka making the right decisions in terms of who to play (Gallo or G Will) depending on matchups which may or may not go well for us. Could be positive, could be negative or more likely somewhere in between.