Gaara's Jinchūriki Forms

Gaara was the jinchūriki of the One-Tailed Shukaku. Although it has an unnaturally large source of chakra compared to most ninja, it isn't as great as the Nine-Tailed Demon Fox, but Shukaku's presence inside Gaara allows him to manipulate sand at will. As an added bonus for Gaara, Shukaku itself can move the sand on its own to protect Gaara from any harm it can in order to preserve its own life. It should be noted that, unlike Naruto Uzumaki's constant battle for control over his body with the Nine-Tails, Gaara is technically never subjugated to this. 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 bloodlust, the latter being a trait Shukaku can easily influence.

Half Shukaku Form
When Gaara was in a rage 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 distorting it and giving it Shukaku's features such as the yellow eye and markings, he even grows his one tail from the gourd which itself functions like a normal tail. In this form, his speed has increased. 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, 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 Shukaku's with the same markings around 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 element to perform Wind Release: Infinite Sand Cloud Great Breakthrough. His speed, attack, and defense greatly rises. In addition much like Naruto's chakra shield, the sand is capable of absorbing a wide variety of attacks. However, the defense 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 Shukaku Form
Technically this isn't a jinchūriki form as its a transformation into Shukaku itself. In desperate situations, Gaara would 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. 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 form, this seems to be less (if at all) 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' color 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. Its full power is never seen as it was destroyed in a short time.

In Naruto Shippuden: Ultimate Ninja Storm 2 he showed far greater control over the Shukaku, able to create the tailed beast's head and fire a One-Tailed Shukaku Menacing Ball, something he couldn't do pre-Timeskip.

Trivia

 * In the games 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.