I've talked about the Gap Theory before on this Blog. Starting with my Fall of Satan post. Where I discussed it's ancient's precedents. But the key difference between the Ancient and Medieval versions, and the more Pseudo-Scientific version that emerged in the late 18th Century. Is where the Gap is.
The idea of a Gap between Genesis 1:1 and 1:2 you won't find in any pre-modern concepts. Instead the Gap was usually placed between Genesis 2:3 and 2:4. It's tied to the idea that the Adam of Genesis 1 and the Adam of Genesis 2 were not the same Adam.
Sometimes there is an attempt to gain New Testament support for such a doctrine by quoting when Paul called Jesus the Last Adam, saying the first Adam created was the last to enter our world. However Paul was just talking about typology here, Jesus is another Adam because as in Adam all die, so in Jesus all shall live again.
In Kaballah and Gnosticism and other mystical traditions. The Genesis 1 Adam is often called something like Adam Kadmon. He might be a sort of Cosmic Man, or an ancestor of the gods like Izanagi. Philo of Alexandria linked Adam Kadmon to the Logos. But there may also be versions where he's just the progenitor of what we would now call a Pre-Adamic race. Adam Kadmon is also sometimes defined as being the pre-incarnate Messiah.
There are two main philosophical ideas that are important to understanding the Creation of Adam, being made in God's Image, and being made from the Dust of the Earth. But when you make the Adam of Genesis 1 and the Adam of Genesis 2 separate, those two ideas no longer refer to the same creation. Genesis 1's Adam is the Image of God, and Genesis 2's is made from the dust of the earth, with the word for earth being Adamah. This is how the Genesis 1 Adam can become a Cosmic Man, or tie into a Platonic idea that our world is just in inferior replication of a Spiritual realm.
This two Adam theology can also be analogized to Egyptian mythology. Where the Genesis 1 Adam could be identified with Atum, who's name has a phonetic similarity to Adam, and who has some Cosmic Man traits with the Sun being one of his eyes. And every soul was believed to come from his Ka. While the Genesis 2 Adam could be identified with Geb/Keb/Seb, who's name has a similar meaning to Adam, Earth. And who had a son named Seth and another son who was murdered by his brother.
Today the standard Young Earth Creationist approach to explaining how the allegedly separate Creation accounts fit together is to call Genesis 2 starting in verse 4 a more detailed account of part of what happened on day 6. I kind of lean toward that still, but have considered that the day refereed to in Genesis 2:4 could be the Eight Day. But I will never consider any compromise that has death before Adam Sinned in Genesis 3.
This dual Adam theory can also play a role in false belief systems where Jesus does have a Pre-Existence but is a created being, like the Jehovah's Witnesses and Origen. It may not always be brought up in such systems, but the door is there to say that the Genesis 1 Adam is Jesus. And that's the main reason I'm uncomfortable with making Genesis 2 the eight day, because it leaves that door open. But I would never use that Adam was created "male and female" in Genesis 1 as an argument against that, since I have argued Jesus has a Feminine side.
Yet on the other hand I tend to oppose the Pre-Exstence of Human Souls in general as much as I support the Pre-Existence of Jesus. So I also wouldn't like saying Genesis 1 was the creation of Adams' Soul and Genesis 2 of his body. That can also lead to Gnosticism. So that's a second reason to not go with the 8th Day theory.
The idea that Genesis 2 starting in verse 4 is the Eight Day of creation, is one I'd consider only in the context of how that might fit Traducianism. The view that Adam's was the only Soul/Spirit directly Created by God from scratch, just as obviously his Body was the only one directly by God from scratch. And that other souls are formed from the Souls of their parents, just as their bodies are, basically that Souls reproduce like bodies do. And Eve was cloned from Adam. A view I'm kind of leaning towards at the moment.
When studying these issues you'll find the word "Creationism" used with a different meaning, referring not to belief in a literal interpretation of Genesis. But to belief that each individual Soul/Spirit is directly created by God from scratch at the conception or birth (or somewhere in-between) of every individual person.
But either way, Jesus individual Pre-Existence is because He is God, it's part of how He's distinct from other humans. In John 1 The Word created everything, while in Genesis 1 Adam was the last thing created on Day Six.
I want now however to address those who think viewing most of Genesis 2 as more details on Day Six is somehow an absurd torturing of the text. It makes sense when you understand that Genesis was mostly much earlier accounts edited together by Moses. Genesis 25 records the Death and Burial of Abraham before the births of Jacob and Esau, even though the timeline based on math from this same chapter tells use they were born before before Abraham died. Genesis isn't always strictly chronological.
Genesis 2:4 says "These are the generations of the Heavens and the Earth". Occurrences in Genesis of "These are the generation of..." are often taken to indicate a change in the Pre-Mosiac source text, though I think they may not quite account of all of them. There seems to be disagreement on if these verses should mark the end of the previous section of beginning of the next section. Either way, we have one right here where it should be.
What is interesting, if you want to consider how these doctrines might play a role in the End Times deception. Is how the Two Beasts of Revelation may be an attempt to echo two Adams. The second Beast comes from the Earth, and then gives life to the Image of the first Beast. Either way, I have already considered how The Image of the Beast doctrine could tie into an attempt to make "The Antichrist" a counterfeit "Last Adam". In my Which Beast is actually in control post, and my Can Sunday Worship be the Mark of the Beast post, where I explain that if you want to tie the Mark to a day of the Week, it should be the Sixth Day not the first.