Hi,
Basically my belief follows this description to a certain degree, although I dont agree 100% with the wikipedia's article. The main points of the gap theory that I believe is that

1)the gap occurs between verse 1 and 2 or genesis 1
"In the beginning God created the heavens and earth .....GAP OF UNSPECIFIED TIME...and the earth was without form and void"

The theory is that God did not create the world without form and void it became that way, and there are many different Hebrew words which, when translated will point to this. One such word is "was"(above) which can be translated "became"

One of the main Biblical proofs(though there are entire books written about this) is found in the Hebrew translation of "without form and void". Those words are the Hebrew "tohu" and "bohu".

Basically I arrived at the theory from a variety of sources, but you can read it from an expert online here. Book By Arthur Custance Dr. Custance is one of the worlds outstanding scholars in Middle eastern languages,he was one of the first to have been involved in serious cuneiform study under Dr. Meek he is a Christian but he has a lot of scholarly education. I dont even know if he is alive today..