Genesis 1:2 mentioned that "...the earth was without form, and void; and darkness was upon the face of the deep".
Gap theory uses this verse to explain certain mysteries, such as the dinosaurs, mysterious warfare between God and the Devil-such as that of John Milton's fantasy in Paradise Lost, etc.
I would think it is interesting. But I wouldn't agree that the Bible provides sufficient information to indicate any evidence of such theory. Therefore, it could also be said that God had the earth created in a very infant state, which requires decoration and care.
As for a mysterious warfare such as that in Paradise Lost, one can take it as an anthropomorphic interpretation of what actually goes on in the spiritual realm.
And the age of the earth is another story.
http://www.jonahome.net/files/tcrwtjd/
圣经·神学·护教
28、创世记一1～2有没有隔离一段的时间?
答：从语言学方面看，这是有可能的。但不一定要那样解释才可以明白上帝的创造。这件事本身在创世记的注释里面可能有六、七种的理论，这只是其中一种理论：The Gap Theory。这个理论虽然可以解决一些的问题，其中却隐藏了许多困难及更多的问题，所以不一定要采取这个理论。