Graham is looking to have another big year. Gates is on the tail end of his career. But, with the Chargers losing Vjax, I see Rivers looking to Gates as an even bigger safety net than usual.
McGahee isn't the same player he used to be, but he'll get plenty of touches in Denver. He put up decent numbers before Tebow stepped in last year. I imagine that Denver will do a decent amount of running to keep the pressure off Manning, but I think McGahee's upside is pretty small.
Steven Jackson is healthy and will be getting 20 or more touches a game. He's a must start when he's healthy.
There are only a handful of elite tight ends right now, but I'm sure there will be some guys to grab off the waiver wire if Gates flakes out again this year. (Daniels, Celek, Bennett, Fleener, etc..) Same goes for running backs, but a lot of the big handcuffs are usually snatched up.
If it were me, I'd do the trade.