Google's Gmail SMTP server is a free SMTP service which anyone who has a Gmail account can use to send emails.
You can use it with personal emails, or even with your website if you are sending emails for things such as contact forms, newsletter blasts, or notifications.
To use Gmail's SMTP server, you will need the following settings for your outgoing emails,
Outgoing Mail (SMTP) Server: smtp.gmail.com
Use Authentication: Yes
Use Secure Connection: Yes (TLS or SSL depending on your mail client/website SMTP plugin)
Username: your Gmail account (e.g. firstname.lastname@example.org)
Password: your Gmail password
Port: 465 (SSL required) or 587 (TLS required)