There are two variants of the theory:
PASSIVE GAP THEORY (Restitution Creationism, Creatio Ex Materia)
According to this view God simply waited for millions of years after creating the heavens and the earth to complete his creative work.
ACTIVE GAP THEORY (Ruin-Reconstruction)
This Theory holds that the earth was originally created with form, but after a gap of millions of years it became “without form and void”. The idea is that, at the end of these geological ages, a great cataclysm took place on earth, with Satan having rebelled in heaven and many of the angels following him in that rebellion. God, therefore, cast him to the earth, and the earth underwent a great cataclysm, leaving it finally without form and void, and with darkness on the face of the deep, as described in Genesis 1:2.