Why do the Bible and the Epic of Gilgamesh both contain a story about a flood?Which was written first?

A definitive answer to this question is not possible, of course, because we do not always know the motives of the authors of these works, or, for that matter, who the authors were.   We also lack a complete geological or archaeological record.  But two possibilities come to mind.

First, there is a kind of theme in both about God or the gods have not done very well with mankind and having a need to wipe the slate clean and start over.  This is clear in the Bible, but more implicit in Gilgamesh. Thus the point of both stories seems to be that mankind had better get its act together!

Second, while the record is not clear, there is some evidence to suggest that a flood may have occurred somewhere in that part of the world. Since I am neither a geologist nor an archaeologist, it is difficult for me to assess this evidence.  If you decide to do research on this issue, you should be aware that many websites discussing the issue are motivated by a need to "prove" the Bible, which, as a religious undertaking, requires belief, not proof.