I appreciate people who are defending Gaara because they feel he gets too little respect, I have often advocated for him myself. I made it very clear that he (or Onoki) could solo the other 3 Kage with ease. He's an insanely powerful guy, well worth of being discussed in the top 20 type of range... but against Itachi? No.
Itachi has too many ways to beat him, while Gaara is going to struggle alot to even hurt Itachi. Itachi is faster, has multiple one shot moves (most of which Gaara can't stop), is a better fighter from a technical standpoint (analytical, reaction time, tactics, etc). Even Ameratasu, which Gaara can stall a little bit with his sand walls, isn't something you can stop when Itachi goes all out and creates an inferno like this:
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Good luck blocking an etheral sword too. Or Shi Sui's eye that doesn't require eye contact to work, or his one shot genjutsu, or long range genjutsu's and doubles, or his Izanami, etc, etc, to say nothing of his Yasaka beads, which will churn right through Gaara's sand defence (to stop it against Madara he and Onoki's defence combined was barely enough).
As has been explained, pulling Itachi out of Susanoo won't work. Unlike Madara, who was too arrogant to try most of the time, and was testing himself, Itachi won't mess around, and will just make sure Susanoo covers his feet the whole time, which is easy to do. He can also stop himself being pulled out by blocking the sand with the Yata Mirror, so Gaara's sand needs to literally pull him through the Mirror, which it obviously can't do. To be honest it was just plot no jutsu that Madara didn't shield under his feet (as he had shown he could do on previous occassions, like when submerged in Mei's lava), because Kishi needed to show that he had the Rinnegan. If Gaara's one time trick could be effective he would have used it against Madara again, something he never did (assumedly because Madara just defended against it by covering his feet).