That's a reasonable question, and let's pretend for a moment the Gap Theory is correct just for the cause of understanding the Theory. Which is not possible for some!
We know that God did create and fabricated just as you said. But in the Gap theory the entire universe+ is created in Gen. 1:1. We have nothing on how that creation was actually created, we only know what took place in the recreation that took place from Gen. 1:3 and on.
Some scholars believe that God created everything in Gen.1:1, and in 1:2 it's all void and with no form. They say this can't be.
They believe something catastrophic took place to cause it to be this way sometime in the dateless past.
They present the theory that the rebellion of Satan and the angels that deflected with him caused a great war in heaven and some of the original creation was destroyed and everything that God created in the original creation in Gen. 1:1 was thrown out of proportion in disarray.
This all comes from what I explained in post #26, that Moses has used 2 different words in the creation model that are opposites to one another.
It's clear to see that from Gen. 1:3 and on God is not speaking the universe into existence as He did in Gen. 1 by the word "created."
It's also clear to see that Moses is using the word "made" which is the opposite of "created" in that "made" is the opposite of speaking into existence and means to refabricate something with materials that already exist.