It is well known that Tolkien was, first and foremost, a linguist. His familiarity with Germanic languages comes into play more than once- think of Beorn (similar to Norwegian bjørn "bear"). It sticks in my mind that the English word "elf" is derived from an earlier word "alf."
Did Tolkien write at all about naming Gandalf and his thought process there?
The name Gandalf was lifted directly from the Dvergatal, or "Catalogue of Dwarves", in the Völuspá. You may recognise some of the other names here too:
Alþjófr, Dvalinn,
Nár ok Náinn,
Nípingr, Dáinn,
Bífurr, Báfurr,
Bömburr, Nóri,
Án ok Ánarr,
Ái, Mjöðvitnir.Veigr ok Gandalfr,
Vindalfr, Þráinn,
Þekkr ok Þorinn,
Þrór, Litr ok Vitr,
Nár ok Nýráðr,
nú hefi ek dverga,
Reginn ok Ráðsviðr,
rétt um talða.Fíli, Kíli,
Fundinn, Náli,
Hepti, Víli,
Hanarr, Svíorr,
Nár ok Náinn,
Nípingr, Dáinn,(Where "Þ" = English "Th", therefore Þráinn = Thráinn, Þorinn = Thorinn, Þrór = Thrór)
This was openly acknowledged by Tolkien in a December 1937 letter, referenced in the foreword to History of Middle-earth 6.
I don’t much approve of The Hobbit myself, preferring my own mythology (which is just touched on) with its consistent nomenclature – Elrond, Gondolin and Esgaroth have escaped out of it – and organized history, to this rabble of Eddaic-named dwarves out of Volüspá, newfangled hobbits and gollums (invented in an idle hour) and Anglo-Saxon runes.
What is particularly interesting about this is that, in the original drafts of the Hobbit, Gandalf was actually the name of the lead Dwarf, whereas the wizard was named Bladorthin, and it was only in the course of composition that the names changed: first of all Gandalf was dropped from the lead Dwarf, but the wizard remained Bladorthin for a short while, only picking up the name Gandalf a couple of chapters later. This is all covered in the History of the Hobbit.
The name "Gandalf" itself is just Old Norse for "Elf with a Magic wand", and Tolkien used the same meaning in his own works, quoting here from the Istari material in Unfinished Tales:
Mostly he journeyed unwearingly on foot, leaning on a staff; and so he was called among Men of the North Gandalf, “the Elf of the Wand”. For they deemed him (though in error, as has been said) to be of Elven-kind, since he would at times works wonders among them, loving especially the beauty of fire; and yet such marvels he wrought mostly for mirth and delight, and desired not that any should hold him in awe or take his counsels out of fear.
Obviously a more appropriate name for a wizard than for a Dwarf!
Gandalf is derived from the Old Norse and could be a composition of Gandr = staff and Alf = elf. So Gandalf means simply an elf with a staff.
But the physical appearance of Gandalf as we know him from LOTR trilogy have perhaps its origin from the 19th century where romantic painters and writers imagined Oden / Odin, from the Norse mythology, as a lone wanderer. Its counterpart, the character Wotan from the Wagner opera Siegfried also contributed to the appearance.
The previous answers are incomplete.
Tolkien may have heard of several Dark Age Norse named Gandalf.
There was Gandalf Alfgeirsson of the tiny Kingdom of Vingelmark in Southeastern Norway in the 9th Century. https://en.wikipedia.org/wiki/Gandalf_Alfgeirsson
A King Gandalf the Old of an unnamed kingdom had sons who were with King Harold Wartooth at the legendary Battle of Bravellir which allegedly happened in the mid 8th century, or about 733 to 766 AD.
There was a kingdom of Alfheimr ("Elf Home" and/or "Land Between the Rivers") on the border between Southeastern Norway and Sweden in the Dark Ages, whose kings also ruled Vingelmark. The royal family of Alfheimr was allegedly related to elves. And I guess that Elvenhome, a translation of Eldamar in Aman, could be inspired by Alfheimr.
Sigurd Ring, nephew of Harold Wartooth, was said to have married Alfhild, a daughter of King Alf the Old of Alfheimr, or maybe a descendant of him, or maybe a daughter of king Gandalf son of Alfgeir son of Alf the Old. In any case she was supposed to be The King of Elfland's Daughter, to coin a phrase.
Sigurd Ring and Alfhild's son was the legendary Ragnar Lodbrok, whose legendary sons are the reputed ancestors of many people living today. And so many people living today can dubiously trace their ancestry back to King Gandalf of Alfheimr, and perhaps to the elves his family was supposed to be related to.
https://en.wikipedia.org/wiki/%C3%81lfheimr_%28region%29
Cathy Gandolfo was a TV newswoman a in Ph8iladelphia for many years. Gandolfo is said to be:
Italian: from Gandolfo, a Germanic personal name composed of the elements gand ‘spell’ + wulf ‘wolf’.
According to this site, so apparently it is not as similar to Gandalf as I thought. http://genealogy.familyeducation.com/surname-origin/gandolfo
And of course Tolkien probably heard of the Papal summer residence at Castle Gandolfo.
https://en.wikipedia.org/wiki/Castel_Gandolfo
So I certainly have to wonder whether Tolkien ever read about men named Gandalf.
