It uses a subset of the email protocol (which makes it very difficult for governments to block) but it no longer uses an an actual email address to function by default.
Even if someone did use a gmail exclusively for this (you can't use it with an email account you use for normal emails too), everything would be entirely encrypted, and only the app itself would be able to decrypt it (google would not be able to decrypt the messages). But again, no normal user is going to use an actual email address.
You can read more about how it works in their FAQ. But the short version is once you pick a username, it just gives you a QR code or link to send to people, which connects you immediately in an encrypted chat room with no faffing around with emails.