Here is the Gap idea of Genesis 1:
Gen 1:1-2
1 In the beginning God created the Heaven and the earth.
In the Hebrew, "God" is Elohiym, which is about The Triune Godhead, but the word "created" is Hebrew barah in singular. This is pointing directly to the full Godhead of The Father, The Son, and The Holy Spirit, but in singular fashion creating the Heaven and the earth.
"Heaven" is plural in the Hebrew here. Per Job 38, God created Heaven first because when He created the earth, all the "sons of God" (angels) sang for joy.
This also means the 'heaven' called our sky around the earth was also created then at verse 1. Because of this, some simply pass this Genesis 1:1 verse off as just as a type summary of what is explained in the rest of the Gen.1 chapter.
This Genesis 1:1 creation in the beginning was God's PERFECT full ORIGINAL creation. It is right here during this time when Lucifer followed God and God said Lucifer was "perfect in thy ways" (Ezekiel 28 about the "king of Tyrus" used by God as a 'type' for Lucifer in that old world).
This Genesis 1:1 verse is the original creation created by The Word of God that Peter was pointing to with "the world that then was" (2 Peter 3).
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.
KJV
Here the earth already exists, as also those "waters" upon it. We find no idea of the earth being created past the point of Genesis 1:1.
The phrase "without form, and void" is not an accurate translation into English. In the Hebrew it is tohuw va bohuw, and means 'a waste and an undistinguishable ruin' (OT:8414 and OT:922; see your Strong's Exhaustive Concordance).
With Hebrew tohuw, in most Biblical cases it is used for something that has gone into 'vanity', 'emptiness', 'nothing'. It does not mean what went into vanity started out that way; it means it was once good but then went into vanity or corruption. Use of your Strong's Exhaustive Concordance, or The Englishman's Concordance, will show you how Hebrew tohuw is used throughout the Old Testament, likewise with Hebrew bohuw.
One of the strongest Scripture proofs that God did NOT create the earth 'tohuw' is in Isaiah 45:18 where He specifically said He did not create the earth "in vain" (Hebrew tohuw).
What this means for Genesis 1:2 is that the creation is no longer Perfect at this point, that something happened to make it go into a waste state covered by those "waters" in that verse.
In Jeremiah 4 there is another description of the earth being in that state of "without form, and void", which further proves that tohuw va bohuw is about the earth going into 'vanity', in a wasted state because of some major event. This also reveals that GOD is Who placed His ORIGINAL PERFECT creation into this vanity state of "without form, and void" (tohuw va bohuw)...
Jer 4:22-28
22 For My people is foolish, they have not known Me; they are sottish children, and they have none understanding: they are wise to do evil, but to do good they have no knowledge.
At this point in the prophet Jeremiah's day, the children of Israel of the southern kingdom of Judah at Jerusalem/Judea was rebellious against God. And God had already determined to bring upon them Nebuchadnezzar, king of Babylon. So God is getting ready here to give them a little reminder through His prophet Jeremiah, of a destruction God did of old upon the earth...
23 I beheld the earth, and, lo, it was without form, and void; and the heavens, and they had no light.
Wait a minute; what's this "without form, and void" doing here if it means the earth had not yet been formed per Genesis 1:2?? As will be discovered with the next verse, God gave this "without form, and void" along with the next verses of an existing... creation that is destroyed by Him...
24 I beheld the mountains, and, lo, they trembled, and all the hills moved lightly.
Have you ever read Hebrews 12:25-29 where it speaks of how God once shook this earth, but that "Yet once more I shake not the earth only, but also heaven,"? Where's is that future... shaking written you think? In Revelation 6:14 on the 6th Seal. By that example in Hebrews 12, God is showing that He once shook this earth and all the mountains and hills trembled and moved lightly.
Did you know that geographic true North is about 90 miles off of true magnetic North? Aircraft pilots have to correct their compass heading when flying long distance over the earth; "east is least" (6 degrees), and "west is best" (8 degrees), is the old saying. Did God shake the earth off its original axis with that first shaking? And Hebrews 12:25-29 indeed is pointing to a final shaking of this earth on the future "day of the Lord" when that "consuming fire" of 2 Peter 3:10 happens.
Thus already here in Jeremiah 4, with that "without form, and void", and trembling of the earth, God is pointing back to a previous whole-earth full destruction event that He did before, and it included a shaking of this old earth, a shaking on the level of the future one "Yet once more..." that is to occur per Revelation on the day of Christ's future return.
25 I beheld, and, lo, there was no man, and all the birds of the heavens were fled.
The birds having fled automatically suggests those birds existed in the sky beforehand.
26 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.
That idea of the "fruitful place" is about the earth. It became like a "wilderness", empty, nothingness. That's the idea of Hebrew tohuw va bohuw ("without form, and void").
And "all the cities" were broken down at GOD's Presence, and by His fierce anger?? I really get amazed at how some men do everything they can to keep their pet traditions with attempting to explain these "cities" being of this present world so they can place this WHOLE Jer.4 section as during the days of Jeremiah.
But I won't lie to you brethren by doing what those men do with attempting to hide this Truth. God literally destroyed that old world that had "cities", His first original perfect creation, and it was because of Lucifer's rebellion against Him with Lucifer wanting to be GOD and coveting GOD's throne. To this day Satan still covets God's Throne and Jerusalem to go with it.
So there were ancient "cities" that existed in that old world of God's original perfect creation prior... to Lucifer's rebellion? Yes.
In Revelation 12:3-4 we are shown a time when the "red dragon" drew a third of the "stars" (put for angels) to earth with him in rebellion. Per Rev.12:9 the title of "dragon" we are told is another name for Satan. At that time he drew those third of angels into rebellion with him, it shows Satan did that with a world system that had "seven heads and ten horns, and seven crowns" (Rev.12:3-4).
That original beast system of ten horns, seven heads, but only 7 crowns is hard-linked to the time when Satan drew those third of the angels to earth in rebellion with him. Those same men that try to hide this Biblical fact try to associate that with the beast system of Rev.13:1 which is to have only "ten crowns". Won't work. The beast of old with only "seven crowns" was back when Satan first rebelled. So what does that reveal about Lucifer's beast kingdom of old that had only "seven crowns"? Since the Rev.13 beast is to have ten horns, seven heads, and ten crowns, does it means it will involve 'cities' upon the earth that the ten horns as ten kings will reign from? Yes. Likewise it likely was with Lucifer being over a "seven crown" beast kingdom back in the old world before he rebelled.
27 For thus hath the LORD said, The whole land shall be desolate; yet will I not make a full end.
28 For this shall the earth mourn, and the heavens above be black: because I have spoken it, I have purposed it, and will not repent, neither will I turn back from it.
KJV
Past Genesis 1:2, God is actually making a 'restoration' of the earth that He placed in a waste and ruin state with waters of a flood at Genesis 1:2. This restoration is our present world. Those waters at Genesis 1:2 upon the existing earth were not about the time of Noah. Noah's time was much later than Gen.1:2 that should be obvious.