1. Adam's fall brings death into the world
2. Gap creationism says that there was a world and beings BEFORE Adam was created
Ergo -- did those beings die? If yes, then how? Death entered the world with Adam's sin, right?
And don't forget that after God created, He said that His Creation was "very good". If death is an enemy that Christ came to defeat, then how is death part of what is "very good"?
Also, consider Exodus 20:11
For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
Putting a "gap" between the first and second day is extra-biblical. It is an assumption made without support of the text.