Short answers, pulled from the story.
Tolkien derived the name Gandalf from a specific entry in the Old Norse poem Völuspá where a dwarf named Gandálfr translates to staff-elf. This etymological root appears in the character's Khuzdul name Tharkûn meaning Staff-man within the fictional language of the Dwarves.
By 1942 Tolkien decided his work was a translation from Westron making Old Norse names represent names from Dale. Thus Gandalf became an anglicised version of the name Dwarves gave Olórin.
Gandalf fought for two days and nights until the Balrog was defeated after climbing Zirakzigil following eight days pursuing the enemy through tunnels. He died there while his spirit traveled out of thought and time before Gwaihir carried him to Lothlórien.
Humphrey Carpenter said Tolkien recalled buying the postcard entitled Der Berggeist during a holiday in Switzerland in 1911. Manfred Zimmerman discovered the painting by Josef Madlener dates from the mid-1920s.
Norman Shelley voiced Gandalf in BBC Radio dramatisations of The Lord of the Rings between 1955 and 1956. Heron Carvic played him in The Hobbit radio series in 1968 and Bernard Mayes took the role in 1979.