Welcome to the Downs, shia'tan!
A Google search on the topic yielded
this page, which lists Tolkien's aunt as the inspiration for Gandalf. However, it also mentions a character from Norse mythology named Gandalfr, and Merlin of King Arthur legend.
I am inclined to doubt the last, however, because Tolkien pointed out that the tales of King Arthur were French, not British, and for him to use a character from it as inspiration seems... odd.