The Term "God", when not used in conjuction with any religion, is a rather loose term... Is it possible that humans were "created" by another race? Sure. I have no doubts that, some races who are millions of years more advanced, would be capable of terraforming the earth.

If we're assuming the role of God is just a creator of life, or the one who brought life here, then I suppose it would be possible.

But the term of God, All Knowing/Powerful, doesn't strike me as even remotely possible.