"Taking on all arguments for both Gap Theory and the Day-Age Theory, Fields outlines the clear facts and proofs for Young Earth Creationism."
I think the foundation of the error seems to be the necessity of holding to a "Young Earth." We can certainly hold to a literal 6-day creation and also hold that the earth is impossibly old. The ideas are independent. I hold to both. I hold that the earth was created in perfection, sin entered, it was cast into chaos for an indeterminate amount of time, then God started his 6-day re-creation.
For thus says the Lord, who created the heavens (he is God!), who formed the earth and made it (he established it; he did not create it a chaos, he formed it to be inhabited!): “I am the Lord, and there is no other. -Isaiah 45:18 (RSV
When I cover the Gap in my Bible group, I try to give all sides and the reasoning behind the different teachings. But what I haven't heard (and I probably should give this author the benefit of the doubt since I've only read the synopsis) is a complete review of the "First Earth" understanding of Gen 1:1.
What if the coming "new earth" is like that "first earth?" No sea (Rev 21:1), pristine, into which Satan's sin was introduced and, by that sin, chaos? We are of Adam. Adam was formed from the dust of the current earth. We are born "in Adam," and God's revelation for this earth is our primary concern.
With that said and understood, part of that revelation is the rebellion of Lucifer and the chaotic and void earth that resulted. It is useful to understand this. The earth was his dominion. He lost that dominion until Adam and Eve gave it back to him. He is "the god of this age" (2 Cor 4:4).
When I first heard of the Gap Theory, it was dismissed as an excuse to accept evolution as part of creation. Well, I reject evolution as nonsense. The Gap has nothing to do with evolution. Believing the earth may be incalculably old does not accommodate evolution nor does it necessarily contradict a 6-day creation.
A proper understanding purports that the 6-day creation is essentially a re-creation. We do not need a "Young Earth" to reject evolution or any other witness of Genesis or scripture in general.