Gap Theory, to my knowledge, it hangs on a few things and can lead to an institutionalized Christian Racism called “Christian Identity.” The racism comes from people believing that the races other than the White Race were created before Adam and are remnants of an earlier, ruined iteration of Earth.
I am not sure of all the arguments for “Gap Theory” nor all the things it seeks to explain. I do know that much of the backing hinges on the word replenish – God saying to Adam and Eve, go forth and Replenish the Earth.
Today, replenish means refill. That, however, was not always the case. The word used to mean fill and the meaning changed after the King James Bible was written. It was changed by none other than alleged Occultist Francis Bacon, author of the incomplete “The New Atlantis.”
I first learned that on Google but they have since changed their own result on that. m-w.com does still have the definition I am talking about but not the history: