1 In the beginning God created the heavens and the earth.
2 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.
3 And God said, Let there be light: and there was light.
4 And God saw the light, that it was good: and God divided the light from the darkness.
5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
The gap theory is about a 'gap' between v.1 + 2 and v.3 and the rest of the 6 days of creation. The wording in the original Hebrew has ('was without form') as 'became without form, and void'.
This allows for a previous earth which was destroyed. The main part of Genesis 1 is about the new creation and the new creation of mankind.
28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moves upon the earth.
That's why it says replenish - mankind is to replenish the earth.
What then, was the earth before? In my humble opinion, it was populated by the reptilian world, and reptilian supernatural angels. This, for me, solves the various mysteries of the distant past - the fact that you find giant ammonites on the stony beach of Lym Regis (UK) - and why they found plesiosaurs and ichthiosaurs - and not seals, dolphins, rabbits, mixed up in the same strata.
As far as I know, mammals are not found in the same strata as the flood deposits where fish etc are found. Perhaps some mammals, but in the main, no.
Whether their were men on that first world - don't know, but the reptilian angel theory solves the mystery of who built the megalithic sites, such as the pyramids and the surviving walls in Peru and Bolivia - built upon with modern walls over the top of megalithic walls.
For me - I do not believe in Noah's flood. This is because, Australia has marsupials, and has always had marsupials - many more types than now, but the flooding did not kill off all the marsupials, obviously, however, the fossil record shows that Australia once had marsupials as well as some dinosaurs.
And if Noah took on-board two foxes and two rabbits, then on disembarking the ark, the foxes would have eaten the rabbits, and the gerbils and hamsters, and then themselves starve - hence extinction of everything, when you also consider that there were no edible plants on the earth after that supposed flood.
The lifeforms that are on the earth now, such as the New World birds, differing from Old World, and Australian birds, is due to the fact that they were created in situ - they were always there, at least as far as the new six day creation in Genesis, after the total destruction of the old world - by flooding/tsunamis.
The dinosaurs died off, I think, due to flooding and climate change - being cold blooded, they couldn't cope with the new conditions. Most of the destruction was due to global floods, possibly caused by tsunamis - the YECs have that correct, but they go with Noah's flood from about 4000 years ago. Some dinosaurs have been dated to 19 and 16,000 years ago.
Coal was formed when mega-tsunamis covered over forests, burying herds of hadrosaurs complete, fossilizing.
New life would have been created, new orders of life, mainly mammals, birds, whales, dolphins - but as darkness was upon the face of the deep, then whatever powered the light in the old world was switched off, and so nothing would have survived, if that darkness was covering a totally flooded world. All life, including the old forms of crocodiles, snakes, spiders etc. were created again, based on the existing templates of kinds.
20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
Moving creature that has life, is here moving creatures and birds, but there is a distinction made between these kinds and the beast of the earth...
21 And God created great whales, and every living creature that moves, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.
The waters bring worth the living creature according the kinds.
22 And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.
23 And the evening and the morning were the fifth day.
24 And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.
here, in verse 24, the distinction is made - and beast of the earth, is according to his kind - beast of the earth's kind.. what is beast of the earth? - it is not a living creature, in the biblical scenario. It is different to beast of the field, cattle and creeping thing - insects.
25 And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creeps upon the earth after his kind: and God saw that it was good.
the beast of the earth is, imo. reptiles, spiders, snakes, flies - remnant kinds from the old world of reptilians. Spiders and flies are imo. of the old order of life - reptilian life, or life derived from supernatural.
26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creeps upon the earth.
man is made based on the template of mankind - based on the angels of heaven - that is why it says that Elohim made man in his image, but the image is after a man has been saved through Christ - only then is man made in the image of Elohim. In verse 26, mankind is given dominion over the various kinds - except for beast of the earth kind - again, reptiles, spiders, flies, snakes. Beast of the earth is not a creeping thing. But beast of the earth is also derived from the earth.
In Genesis 3, the devil is cursed above the beast of the field - which implies that the beast of the field was also cursed.
27 So God created man in his own image, in the image of God created he him; male and female created he them.