There is no difference between you and God, so existence of one substantiates existence of the other.
Still, eyewitness testimony is enough in the court, so why wouldn't it be in our little debate?
People experienced the existence of Amun Ra, Rama and stuff. I don't think these gods have ever been successfully debunked.
(I do not think people have had personal experiences with Flying Spaghetti Monster, though, but I may be mistaken).
God either exists or doesn't. If you exist, so does Rama, so does Zeus.
Existence cannot be defined. Does being an observer mean that the observation is real, and is the observer a part of the observed? I do not know, and neither do you. This is what I've been trying to point out when I invoked Sancho Panza's cave (or Plato's cave if you like).
I do not know why you think that some gods exist and other do not, or that you exist and God doesn't. Existence either is real or isn't.