Where did God come from? If we decide this is an unanswerable question why not save a step and conclude that the origin of the universe is an unanswerable question.
Everything starts somewhere, though many physicists disagree. But people have always been dimly aware of the problem with the start of things. They wonder how the snowplough driver gets to work, or how the makers of dictionaries look up the spelling of words.
"In the beginning God created the heaven and the earth..." Whatever our speculations may be in regard to a "beginning," and when it was, it is written in the rocks that, like the animals and plants upon its surface, the earth itself grew.
Because there is a law such as gravity, the Universe can and will create itself from nothing. Spontaneous creation is the reason there is something rather than nothing, why the Universe exists, why we exist. It is not necessary to invoke God to light the blue touch paper and set the Universe going.
Initial of Creation, and The Exponent of Earth