Gandiva is the strongest bow supposed to be unbreakable. Along with the bow was two quivers of arrows. The arrows in the quivers was said to never get exhaust. Thus the archer could keep up with a constant and relentless shower of arrows from Gandiva. The string of the bow was so tied that if twanged it sounded like thunder, creating terror in the heart of the enemies
Gandiva was created by Chaturmasa Brahma to protect Dharma. He gave it to Prajapati Daksha. Daksha handed over it to Chandra, the Moon God. Chandra gave it to Varuna, the Sea God.
Meanwhile it so happened that Agni, the Fire God was hungry and wanted to devour the Khandava Vana, the dried forest.
In that forest lived Takshaka, the King of Serpents. He was very close to Indra, King of the Devas. So every time Agni tried to burn down the forest, Indra would ask Varuna to drench it down toake sure Takshaka was safe.
Unable to satisfy his hunger Agni approached Arjuna to help him burn down the Khandava Vana. It is important to note that the burning of Khandava Vana was to rejuvenate the vegetation by burning out the dried up forest.
Sri Krishna siezing the opportunity tells Agni to fetch the Gandiva so that Arjuna can confront Indra. Agni explained the situation to Varuna who hands over Gandiva and the quivers to Agni. Anyhow the purpose of creating the bow by Brahma had to materialize.
Agni offered them to Arjuna. Arjuna was overjoyed. The mighty bow was now his! However using the bow was not an issue to Arjuna as the bow displayed its might only when it bonded well with the archer who mends it. Gandiva bonded with Arjuna very well. Arjuna helped Agni burn down the forest with the Gandiva by winning over Indra.
It is said that despite having many guardians, Gandiva truly became a part of Arjuna, it belonged to Arjuna, the way it never belonged to any of its previous custodians... In that sense Arjuna was one with the Gandiva. Even now Gandiva is associated mostly only with Arjuna.
However after the end of the Kurukshetra war, the Gandiva had served its purpose. Agni asked Arjuna to return the Gandiva back to Varuna as he no longer needed it. Realizing that its purpose was over, Arjuna returned the Gandiva back to Varuna, with whom, it is believed the Gandiva still with.