Gonzo actually had the best fantasy season last year of any tight end. It'll be interesting to see what happens to Witten now that TO is gone. He could become more valuable because Tony Romo will have to throw to him more of he could decrease as a result of defenses focusing more on him now.
If Gates is healthy he has more upside than both of them.
Either way I'd keep Witten over Marshall. His team downgraded at HC and QB big-time this off-season plus if he ends up getting traded it will kill his fantasy value b/c he'll have to learn new plays.