This is a tough matchup that can go either way but I think Tobirama and Gaara can high diff or extreme high diff. They have an answer for most of Itachi's arsenal.
- FTG and sand defense to deal with amaterasu.
- Partner method and clones to deal with genjutsu.
- Any susanoo attacks are dodged by Tobirama.
Although I don't think Gaara is pulling Itachi out of his susanoo since Tobirama shouldn't be much of distraction since nothing in his arsenal can bypass susanoo so Itachi should be able to dodge Gaara's sand. If Minato gets a kunai near Gaara though he can take him out with a rasengan and a lot of Gaara's attacks can be countered with Minato's barrier seal.
I'm actually not so sure about who wins this. It can really go either way.