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.