Gaara's Jinchūriki Forms

Gaara was the jinchūriki of Shukaku — the One-Tail. The beast has a vast amount of chakra, even more so when compared to most humans. Shukaku's presence inside Gaara allowed him to manipulate sand at will. It was initially thought that Gaara's automatic Shield of Sand stemmed from Shukaku and the beast's own self-preservation instincts. However both Gaara's father and uncle stated that it was in fact his mother's will that moved the sand after she swore an oath to protect him no matter what before dying. Because Shukaku is said to devour the soul of its jinchūriki if they were to fall asleep, Gaara has spent up to fifteen years not sleeping at all and when the series started he was driven mostly from a desire for his own existence, and blood-lust, the latter being a trait Shukaku can easily influence.

Half Shukaku Form
When Gaara became angry after a fight with Sasuke Uchiha, he slowly slipped into his half Shukaku form. In this form the gourd on his back slowly breaks down and begins to cover his body, covering his arms, and then his face. His face becomes distorted and it begins to look more like Shukaku's with yellow eyes and markings around the eyes and he grows a tail from the gourd. In this form, his speed and strength increase. He can use the sand on his arms to stretch his limbs to extraordinary lengths. This form allows him to even fire off his limbs at will, but still have them remain under his control, as he was able to pin Sakura Haruno to a tree and then slowly crush her as his battle with Naruto continued.

Miniature Shukaku Form
During his fight with Naruto Uzumaki, Gaara allowed the sand to cover his entire upper body, with only his feet remaining uncovered. While his facial features still remain slightly human, both his eyes resemble the Shukaku's. His body also has the same markings that Shukaku has on its body. Notably, the "love" kanji remains on his head. In this form, Gaara is able to use the Sand Shuriken, and can also use Shukaku's Wind Release to perform Wind Release: Infinite Sand Cloud — Great Breakthrough. His speed, attack, and defence greatly rises while he is in this form. Much like Naruto's fox shaped chakra cloak, the sand is capable of absorbing a wide variety of attacks. However, the defence given to his body still has one weak spot. Since his lower half is not covered, as seen by Naruto, a small explosion beneath his tail can make the entire sand form crumble, and fall apart. This was Gaara's downfall during his battle with Naruto.

Full One-Tail Form
Gaara could use his sand to instantly create a giant copy of Shukaku with himself hidden inside. While he is awake, Gaara still has control of Shukaku's body and can fight "normally" with it. He first turned into this form after Yashamaru tricked him into believing his mother never loved him due to a test his father set to see how well he could control the beast. While in this form, Gaara transfers his eyesight to that of Shukaku's body in turn resulting in his own eyes losing their pupils. Unlike his smaller Shukaku forms, this form seems to not be able to reform itself if damaged. For example, Gamabunta cut off one of the Shukaku's arms and it wasn't able to reform.

When Gaara needs Shukaku's full power, he would expose his upper body on top of Shukaku's head and use the Feigning Sleep Technique, which allows Shukaku to take full control over its body. While taking over the body, Shukaku's eyes will become more apparent, changing the irides' colour from light brown to yellow. Shukaku can use Wind Release: Drilling Air Bullet, by pressing compressed air out of it's stomach, which can cause massive damage and tear down a large line of trees instantly, even dealing significant damage to Gamabunta.

The major weak spot of the full Shukaku form is as expected by Gaara himself being Shukaku's chakra that keeps the large creation going. When Naruto successfully hit Gaara, he woke him, stopping Shukaku's increase in power as Gamabunta stated the longer Gaara was asleep, the more of Shukaku's real power manifested and if asleep too long the Shukaku will eat away at the host's psyche until it takes complete control. After Gaara was knocked out, the entire Shukaku copy was shattered, dissolving back into sand. Its full power is never seen as it was destroyed in a short time.

In Naruto Shippūden: Ultimate Ninja Storm 2, he showed far greater control over the Shukaku, able to create the tailed beast's head and fire a Tailed Beast Ball, something he was not able to do in the past.

Trivia

 * In Naruto: Ultimate Ninja 3, Naruto Shippūden: Ultimate Ninja 4, and Naruto Shippūden: Ultimate Ninja 5, Gaara's miniature Shukaku form is playable as a transformation.
 * Gaara's jinchūriki forms are noticeably different from the others in that it's not made of pure chakra, but of sand. It is, however, possible that the forms that Gaara has shown are partial transformations.
 * In Naruto: Ultimate Ninja and Naruto: Ultimate Ninja 2, Gaara's half-Shukaku form is only playable when he uses his primary ninjutsu and secondary ninjutsu.