Ga = "giga-annum" - billions of years. A mythological unit of time not much different from the "in illo tempore" (once upon a time) of the fairy tales.
How can ‘one billion years’ be a mythological unit of time?
Even if you argue that the Universe is not that old do keep in mind that God is Eternal and Immemorial and that God has always existed.
He existed one billion years ago and He existed a billion billion years ago. He will exist into time beyond our comprehension or our ability to measure.
For our God one billion years is not mythological, it is but a footnote.