Any belief about the origins of the observable universe rest on a leap of faith. It doesn't matter if it's God or a quantum fluctuation, there is no way to know about anything prior to the origin of the universe (assuming there is one). So it's not a "god of the gaps" explanation, since there is not actually a gap there.

If you claim that the cause of the observable universe is a natural phenomenon like quantum mechanics, then it necessarily raises the question "where did the laws of quantum mechanics come from?"

The idea that quantum mechanics simply exist for no reason at all is no more plausible than the idea of a creator.