Well, I have to commend ACB for trying to teach the Gap idea from God's Word. Some of his wordage might be misleading, but the core is very Biblical.
1. Try and find just when it was when Satan first rebelled against God, coveting His throne. The only Biblical evidence for when that was in parable form. So if the Bible student doesn't even understand that, then they certainly won't understand about the Gap.
2. The KJV translation of "without form, and void" is actually misunderstood. Man's scientific theories are immediately applied to that phrase when in the Hebrew (tohuw = without form) the meaning is most often about something originally in a good state going into ruin. Our Bible teachers jump immediately to man's theories of science, telling us it means God had not yet created the earth at this point, that the earth was formless, a nothingness state, like gaseous ether floating in space, which is how science explains the beginning of the origin of a planet.
3. The Jeremiah 4:23 repeat of the earth being "without form, and void" is disregarded by those against the Gap idea. The Jeremiah 4:23-28 is a version of the Genesis 1:2 event that shows the earth had gone into a waste state, God having destroyed a previous time.
4. In Jeremiah 4:28, God says the earth shall "mourn, and the heavens above be black" because of His destruction upon the earth in that time. He said He would not make a full end though. How does He mean the earth will mourn because of that old destruction?
5. In Romans 8, Apostle Paul explains that the 'creation' was placed in bondage of corruption (rendered "creature" in most of Rom.8, but is same Greek word as "creation" Paul used in a later Rom.8 verse).
19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
20 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,
21 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
So when did God place the creation in bondage to corruption? I thought in the beginning God made a perfect creation that we still see today? That is what we have been taught, and even what the secular world believes, i.e., that today's condition of the earth is the 'original' one. It is not, and Paul showed it there in Rom.8 by revealing how the creation was placed in corruption seeked a release from it:
22 For we know that the whole creation groaneth and travaileth in pain together until now.
There's the connection to Jer.4:28 about the earth mourning, and the heavens above being black. What do black clouds show? They're rain clouds, full of water. Most of our violent storm weather is because the sun heating up the air on the ground and warm air rising to mix with cold air and thus cause the winds, storms, etc. We think that is a natural condition, and that God created the earth's weather system to be that way. He didn't, which is why at the end of Revelation He showed there will be no more sea. Where will all the waters on the earth go, since the oceans make almost 3/5ths of the earth's surface?
The reality is, that today's state of the earth is not... how God originally created it. It is not a perfect creation. Even the difference with carnivorous-like animals no longer attacking each other in His future new heavens and a new earth ought to be an eye opener to how His original perfect creation once was, before He placed the creation in bondage of corruption (Isaiah 11).