Setting up Gmail email is done via SMTP.
Username: Your email address
Password: The password to your email address
From Email: Your email address
The last thing you need to do before you are able to send emails via Gmail is visiting this link: https://myaccount.google.com/lesssecureapps while signed into your Gmail account you intend to you. Please make sure Allow Less Secure Apps is toggled to "ON" as that will make sure the photo booth can connect to your Gmail.
After you have that in, you can click save and start to load the photobooth software.
These settings need to be followed exactly to be able to work. Some Gmail emails may not be able to turn off less secure apps or may have problems with the port on the server, if this is the case it will not work.