The answer is:
His raiment was the color of the king of Doriath's cloak,
Grey (when he was Gandalf the Grey). More detail: Elu Thingol is the king of Doriath, and Thingol is a Sindarin word coming from sindacollo meaning "grey cloak".
But now is the color of the Spirit of Fire's prized creations.
White (he became Gandalf the White). The Spirit of Fire is Feanor, his creations being the white Silmarils.
His weapon is from that which was betrayed by the Child of Twilight.
Glamdring (Gandalf's sword), which was forged in Gondolin. "Child of Twilight" is a translation of the name not given to Maeglin, who betrayed the location of Gondolin to her enemies. Thanks to Paul Rowe for help with this clue.
His lords far out number his peers-
There are a lot more Valar (lords of the Wizards) in Middle-Earth than Maiar (Wizards). Thanks to Spacemonkey for help with this clue.
-Them being only the Giver of Gifts and Aiwendil.
The Giver of Gifts is Saruman; Aiwendil is Radagast. (You've forgotten the two Blue Wizards out east though!)
Now who could it be, tell me that?
And may the wind under your wings bear you
Where the sun sails and the moon walks.
“Farewell," they cried, "Wherever you fare till your eyries receive you at the journey's end!" That is the polite thing to say among eagles.
"May the wind under your wings bear you where the sun sails and the moon walks," answered Gandalf, who knew the correct reply.