The principle behind the Gongen is interesting.
It basically means that a shintoist kami (a ghost literally, but it means god. Gods in shintoism can be weak or strong, big and small) can be the manifestation (the avatar as you say, which is pretty accurate) of a Buddha.
So it is basically an interface between two religions, almost a unification, which is quite fascinating and pretty unique as far as I know.
It is something that cannot happen in most religions as pretty much all of them claim to be the only correct one and disallow polytheism.