Gallo's the only one who I don't consider "untouchable" on your list. I think he can develop into a solid player, possibly even a lead scorer, but he's not going to be able to do that on a team with Melo/Amar'e/Felton. Not when he can't hit open 3's, and his biggest attribute (drawing fouls) will not even be needed when Melo and Amar'e can both draw fouls at a similar rate.
Unless you're looking to hold onto Gallo because he has more value, so we can later flip him for something more useful, I'd much rather keep Fields. For me, Fields and Mozgov are the players I'd want to keep in any deal for Melo (no one should be untouchable though, but if Denver wants one of those two, then we should pull others like Gallo/Chandler off the trade table).
There's something valuable to be had in having our starting 5 covered immediately -- Felton / Fields / Melo / Amar'e / Mozgov. Look at Boston for example. Once they acquired KG/Allen, they've had the same starting unit from the very beginning, and through playing together, have only become more cohesive as a unit. We would then need to work on bench depth, but we'd have a mid 1st in 2011, Douglas, Williams, and Turiaf as a starting point, which is really not far off from what Boston had in year 1 anyway.