Fictional biography change
Gandalf the Grey wanted Bilbo Baggins to let him go an adventure with him and the Company of Thorin Oakenshield to take back Erebor from Smaug. Decades later, he would help Bilbo's nephew Frodo bring the One Ring back to the fires of Mount Doom.
Peter Jackson adaptations (2001–2003, 2012–2014) change
In the Peter Jackson movie adaptations of both books, Gandalf was played by Ian McKellen. In The Hobbit movies, which were written as a prequel to The Lord of the Rings movies, he figures out that the Necromancer happens to be Sauron.
His notable quote "You shall not pass" is known for fantasy and literary fans.