Where was Shukaku when Naruto was taken to the Bijuus' Deep Psyche?

Question

At the start of Naruto's fight with Tobi during the Fourth Shinobi World War, Naruto attempted to remove the chakra stakes from the bodies of the Bijuu (tailed beasts), and was taken into the Bijuus' Deep Psyche, where Bijuus and Jinchirikis were present.

However, the following were missing:

1. Son Goku, whom Naruto had already met, had been absorbed by Tobi into the Gedo Mazo.
2. Gyūki, who was still inside Bee.
3. Shukaku. I have no clue of his whereabouts, and I don't remember he joined the fight. Where was he?

If he had been present there, Naruto could have taken the chakra from Shukaku, and used it for the "tug of war" that he got into later during the Juubi Revival arc.

Answer 1

Shukaku remained sealed in the Gedo Mazo. It was the only Bijuu that Tobi did not reseal into its (reincarnated) former Jinchuriki, for his version of the Six Paths of Pain technique. He needed only six Jinchuriki-Bijuu pairs for this technique, while Akatsuki had sealed seven into the Gedo Mazo, so someone had to be left behind.

Moreover, Shukaku's known former Jinchuriki, Gaara, was still alive. Its previous two Jinchurikis were failed experiments according to Chiyo, so Tobi/Kabuto may have decided it was not worth the trouble trying to reincarnate them with Edo Tensei.

Tobi's version of Six Paths of Pain