You can sign up for Flood using your Google account by clicking the "Sign up with Google" option on the signup page.
After you have signed in to your Google account, you'll be redirected to Flood and prompted with this popup:
If you signed up for Flood with a traditional way (using email and password), but later realised that your email is also a Google email, you can later sign in to Flood using your Google account by clicking the "Login with Google" option on the sign-in page.
Follow the prompt to log in to your Google account. After authenticated, you'll be redirected to Flood.