The Celtics don't control Gallon's rights. He's not on our 15 man roster, and if we wanted to play Gallon, we wouldn't "bring him up" -- we'd have to sign him as a free agent. To do so, we'd have to cut one of our other 15 players (i.e., we couldn't just send Bradley to Maine to take his place).
Think of Gallon as a free agent that just happens to play for our minor league affiliate. Yes, we got to filter him there due to the new D-League rules about player allocation, but he has no further obligation to the Celtics.