As lobsterghost said, there is no way you can trace someone yourself. And the police would need to get a court order to try and get data from the email provider, and with Gmail finding the sender will be almost impossible.
I wish I could be more encouraging and ease your fears, but all I can say is that if this message was rather generic - that is, it didn't contain details about you personally - then it was quite probably just a random prank (if one can call a threatening message a "prank") like swatting incidents and not actually a valid threat.
But you do need to involve the police so that they can help evaluate the actual level of threat.
Regards.