I don't think Gandalf will be redirecting a Killing Curse any time soon. Istari magic isn't like Potter's. There are no energy beams "sizzling through the air" in Middle-Earth. It's more alchemy and divinity-driven magic, like Gandalf's "Flame of Arnor" and that light he used against the Nazgul. The speed and lethality that Voldemort can unleash is too much.