Eru Ilúvatar
As one of the Maiar, Gandalf was not a mortal Man but an angelic being who had taken human form. As one of those spirits, Olórin was in service to the Creator (Eru Ilúvatar) and the Creator's 'Secret Fire'. Along with the other Maiar who entered into the world as the five Wizards, he took on the specific form of an aged old man as a sign of his humility. The role of the wizards was to advise and counsel but never to attempt to match Sauron's strength with their own, and hopefully the kings and lords of Middle-earth would be more receptive to the advice of a humble old man than a more glorious form giving them direct commands.