Dumbledore and Grindelward first met when they introduced to each other by Bathilda Bagshot when they were seventeen. As they had much in common, they got along instantly. They both agreed on ideals about the structure of the wizarding society and were both keen on finding the Deathly Hallows. The two grew close and Dumbledore began to grow more romantic feelings towards his friend, but his feelings were unrequited and indeed probably unrevealed, given the culture of the early 20th century, but their friendship remained close.
However, Grindelwald was a sociopath, and may have sensed that the depth of his friend's affection for him could be used to control his emotions and stop him from reaching his aims. They remained friendly for two months, making plans to find the Deathly Hallows and lead a wizarding revolution but Albus neglected his younger siblings, Aberforth and Ariana. When Aberforth confronted the pair over this, Grindelwald lost his temper and attacked Aberforth. Albus rushed forward to defend his brother, and during the duel Ariana was killed. Grindelwald fled, his friendship with Albus over.
Over the next years, Grindelward caused chaos throughout the wizarding world based on his beliefs about their society. Dumbledore refused to face him out of fear that Grindelward would blame him for Ariana's death. Eventually, Albus could stand no more with Grindelward reign of terror. He confronted him in a duel where Dumbledore managed to win. The duel would forever be considered one of the most epic duels in the wizarding world. Dumbledore took possession of the Elder Wand and imprisoned Grindelwald in his own prison Nurmengard.
Many years later, Voldemort confronted Grindleword and asked for information about the Elder Wand. Grindleward who was imprisoned and wandless to the famous dark wizard, refused to give any information. This seems to indicate that Grindelwald no longer held with the views of those who practised the Dark Arts. It was speculated by Harry Potter that Grindelwald had given his life to prevent Voldemort from desecrating Dumbledore's tomb, perhaps indicating that Grindelwald retained some respect and affection for his old friend. However, it is unknown if he ever had any romantic feelings for Albus.
This pairing had a small popularity before J.K. Rowling announced in 2007 that Albus Dumbledore is gay with a romantic attraction to Gellert Grindleward. Although, many fans embraced the idea of Dumbledore's sexuality, many also wished that Rowling had included his sexuality and relationship with Grindleward in the books. After the announcement, more fanfiction and fanart were produced on the pairing. Many displayed Dumbledore's life as a teenage boy with exploration into the relationships between Albus, Gellert, Aberforth and Ariana. Also, the pairing was written with Grindleward returning his feelings to Dumbledore and the eventual duel between both of them.
Notable Fanfiction, Communities and Resources