The Gap theory teaches a Gap between Genesis 1:1 and 1:2. Consider below.
Genesis 1:1-2  In the beginning God created the heaven and the earth.  And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
What Gap theorists believe happened during this Gap is that there was a world, just as ours, in which the rebellion of Lucifer took place, and the earth underwent a major cataclysmic event called the Luciferian flood. After which, God re-created the world in six days.
Is this biblical? ABSOLUTELY NOT. Below is a photo that perfectly describes why it is not biblical.
Did you get it? If you didn’t get it, then it is because there is absolutely nothing in scripture to support it. The argument used by Gap theorists is that Satan was already on earth before the fall of man, so the rebellion of Lucifer must have taken place before the alleged gap. That idea is also not biblical. Here is why.
Genesis 1:31 And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day.
The text says that EVERYTHING God made was good, and that means the Luciferian rebellion could not have taken place yet. Lucifer’s rebellion causing him to fall to earth and become Satan, must have taken place between the end of creation, and the point when the fruit was eaten.
There is no text in scripture to support the gap theory. If it really did happen, God most certainly would have told us so. In the name of The Lord Jesus Christ I write unto you. Amen.