Gaara, like Naruto Uzumaki, is a Jinchūriki of the One-Tailed Shukaku. Unlike the Nine-Tailed Demon Fox, Shukaku doesn't have a unnaturally large supply of chakra that Gaara can use. Instead, Shukaku's presence inside Gaara allows him to manipulate sand at will. As a 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 however, unlike Naruto's constant battle for control over his body with the demon fox, 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 upwards to 15 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.
Controlled Jinchūriki Form
Though not in the same level as Naruto's Jinchūriki Form, Gaara's Jinchūriki Form is more or less simply him in control of his body and power. To this end, he can manipulate any grain of sand, but is easier if it has been mixed with his chakra. If need be, he can even create more sand by pulling ore from the earth and crushing it to make sand. The defining feature of this and all of his forms is that the sand has a mind and will on its own. Without even wanting it, his body is constantly covered in the Shield of Sand, an automatic shield to protect him.
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 and jumps have 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, Gaara's "愛" remains on his head.
In this form, Gaara is able to use Sand Shuriken with apparent great ease as most of his body is now sand. He can use Shukaku's wind element as he could now use the Wind Release: Infinite Sand Cloud Great Breakthrough. 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.
Full Shukaku Form
Technically this isn't a Jinchūriki form as its a transformation into Shukaku itself. After being hit by Naruto's Uzumaki Naruto Two Thousand Combo and moments away from being hit by a second, Gaara would in desperation, 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 loosing their pupils.
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 apparant, changing from a simple light brown pupil, to its original version of a yellow pupil with a small black star with four blue circles around it. Shukaku can use Wind Release: Drilling Air Bullet, by pressing compressed air out of it's stomach, to cause major blunt damage to its opponents.
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 punched 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. After Gaara was knocked out, the entire Shukaku copy was shattered. Its full power is never seen as it was fortunately destroyed in a short time.