Without adding an extention, the best way is to add and configure PostFix for Gmail account (Tested on Mac & Linux):
Connecting to the Gmail SMTP server requires both SSL and authentication. To set up authentication you will need to edit the /etc/postfix/sasl_passwd file.
sudo vim /etc/postfix/sasl_passwd
Update the contents to include the following. Note that enclosing hostnames with square brackets –  – tells Postfix to avoid doing an MX lookup. Make sure to replace EMAIL with your email address, and PASSWORD with your Gmail password, properly escaping any colons in it.
Use the postmap command to update the SASL credentials in Postfix :
sudo postmap /etc/postfix/sasl_passwd
Postfix Relay Configuration
Next we need to edit the Postfix configuration found in
sudo vim /etc/postfix/main.cf
I was not able to route mail to the Gmail SMTP servers over IPv6, so force only IPv4 connections by searching for the inet_protocols key and updating the value.
inet_protocols = ipv4
If any of the following keys already exist in your configuration comment them out, and include the following at the bottom of the config file.
Gmail SMTP relay
relayhost = [smtp.gmail.com]:587 # Enable SASL authentication in the Postfix SMTP client. smtpd_sasl_auth_enable = yes smtp_sasl_auth_enable = yes smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd smtp_sasl_security_options = smtp_sasl_mechanism_filter = AUTH LOGIN # Enable Transport Layer Security (TLS), i.e. SSL. smtp_use_tls = yes smtp_tls_security_level = encrypt tls_random_source = dev:/dev/urandom
Restart Postfix & Test
Restart Postfix as root, then send a test email.
sudo postfix stop && sudo postfix start date | mail -s "Test Email" YOUR_EMAIL@gmail.com
Magento 2 send via PHP Email by default. If you want to use Gmail to send email, you should try to install Custom SMTP module. I found this SMTP module on Github - Yes, Free :)
Hmm, a screenshot: