As a person who still plays Gamma World 1E (Yes, the original, that came in a box, with the One Paperback Book and the huge, cartoonish dice) this just sounds silly.
I mean, the DnD 4E Mechanics to me, just don't fit the Gamma World game, and there isn't 'magic' in Gamma World. I didn't even think d20 Modern worked with Gamma World. There are, as others have mentioned, so many other DnD Settings that should probably merit consideration way before Gamma World would even be mentioned. Gamma World has a very core, dedicated following, and successive incarnations of the game haven't fared well.