Just to give a basic definition of Gap Theory....this is from Wikipedia:
"Gap Creationists believe that science has proven beyond reasonable doubt that the Earth is in fact far older than can be accounted for by merely adding up the ages of Biblical patriarchs, as given in the Book of Genesis. By taking the age of each father at the birth of his son, and adding the "six days" of creation, Young Earth Creationists thereby arrive at an age for the earth, concluding that the Earth is only 6,000 - 7,000 years old. In order to hold the two seemingly-contradictory viewpoints that the Bible is inerrant in all matters of fact as well as faith and doctrine and that the Earth is very ancient, they must account for the supposition that certain facts about both the human past and the age of the Earth have been omitted from the Biblical account rather than falsified by it."
The problem is the 2 views are entirely incompatible even if you try to avoid the "Age" issue. If you add the gap, of millions or billions of years, between day 7 and the fall...then you have a world of death and disease before sin. (”by one man sin entered into the world, and death by sin’ (Romans 5:12))
From a scientific standpoint, it also flies in the face of uniformitarianism....which is the "continuity of processes in the ancient world with those in the modern world".