That's between your browser and their server. I'm referring to the content of the SMTP connection over which the mail travels, which remains almost always unencrypted in the modern world. Which essentially means that the FBI doesn't need Google's assistance to wiretap your email per se, they just use and machine they probably have sitting on the backbone pipe anyway.
And again, that's between the client and the server. The path between Gmail's mail server and the sender/recipient of the mail's server is not always encrypted.
