postfix use Gmail as a relay host.

Undo all your changes to master.cf.

In main.cf, add these settings:

relayhost = [smtp.gmail.com]:587

smtp_tls_security_level = verify
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous

In /etc/postfix/sasl_passwd, add your Gmail username and password, like this:

[smtp.gmail.com]:587    user@gmail.com:mypassword

Run

postmap /etc/postfix/sasl_passwd
postfix reload