I have no problem with the gap theory or the day-age theory as interpretations of Genesis 1, but I take issue with the idea that there was a civilization on earth before it was void and without form. Namely because there is no scripture to support it.
One of the passages of scripture that deal with the earth before the destruction is Jeremiah 4:23-26: I beheld the earth, and, lo, it was without form, and void; and the heavens, and they had no light. I beheld the mountains, and, lo, they trembled, and all the hills moved lightly. I beheld, and, lo, there was no man, and all the birds of the heavens were fled. I beheld, and, lo, the fruitful place was a wilderness, and all the cities thereof were broken down at the presence of the LORD, and by his fierce anger.There was no "man", or Adam is the literal. He was not around yet. Yet, we read of this catastrophe and beforehand it was compared to a fruitful place with cities. We must then wonder who or what creatures lived there before this destruction took place. In any bible footnote, you will see this passage alluded to when reading Genesis version "without form and void".