The Third Kazekage studied the abilities of a previous jinchūriki of Shukaku and learned to imitate the power to control sand. However, when he imitated sand techniques, he incorporated iron powder instead with his magnetic powers. He could use it to crush his opponents or mould it into weapons. In Gaara Hiden, Kajūra, a terrorist from Ishigakure, can also use Iron Sand.
Rasa from Sunagakure could manipulate Gold Dust with his magnetic powers, which is specifically used to counter Shukaku's sand, taking advantage of gold's heavier weight by mixing it in with the sand to hamper its control. When Rasa uses this nature, he gains black rings around his eyes, similar to the ones around Gaara's eyes.
Toroi from Kumogakure would magnetise special shuriken. When the thrown shuriken comes into contact with any other object, it causes the struck target to become magnetised. One hit to the body will cause a magnetic field within said body, making it an easy target for any further magnetised weapons to be drawn to it. However, each time the magnetism is transferred, it weakens.
The anime-exclusive character Jiga also possessed a seemingly similar ability of magnetism. This was from ingesting iron when he was younger which essentially made him a human magnet.
This is so far the only nature transformation that does not use the element for direct offence. Instead, all the users so far use it to manipulate weapons to attack as the chakra is converted into magnetic forces, rather than metal itself.
Magnet Release has been used by at least two of the five Kazekage of Sunagakure. It's possible that this chakra nature is not genetic, as the fourth databook states that Rasa learned the Third Kazekage's Magnet Release.
The magnetisation of Gold Dust and manifestation of rings around the eyes by the Fourth Kazekage may indicate a diamagnetic strain of Magnet Release that magnetises inherently non-magnetic materials like gold and sand.
In Japanese folklore, tanuki are widely known for their ringed eyes. Shukaku is such a creature.
Though originally attributed to his insomnia, Gaara's ringed eyes are possibly the result of his "Absolute Defence" (絶対防御, Zettai Bōgyo) being in constant effect.
References and Notes
↑Rasa has the Wind, Water and Earth Release. Wind and water creates ice, water and earth creates wood, so the only possible combination left for Magnet Release is Wind and Earth Release.