[Narrator: "It wasn't."]
The 1.5.6 fix added Gmail for upgraders from 1.5.5 or prior but not for new installers.
This time for sure! Fixed in 1.5.8. PR: https://github.com/zencart/zencart/pull/3923
Users who need the Gmail value to show up in their Email Transport options list now should use this SQL update in Admin > Tools > Install SQL Patches:
DELETE FROM configuration WHERE configuration_key = 'EMAIL_TRANSPORT';
INSERT INTO configuration (configuration_title, configuration_key, configuration_value, configuration_description, configuration_group_id, sort_order, set_function, date_added) VALUES ('E-Mail Transport Method', 'EMAIL_TRANSPORT', 'PHP', 'Defines the method for sending mail.<br><br><strong>PHP</strong> is the default, and uses built-in PHP wrappers for processing.<br><strong>smtpauth</strong> should be used by most sites, as it provides secure sending of authenticated email. You must also configure your smtpauth settings in the appropriate fields in this admin section.<br><strong>Gmail</strong> is used for sending emails using the Google mail service, and requires the [less secure] setting enabled in your gmail account.<br><strong>sendmail</strong> is for linux/unix hosts using the sendmail program on the server<br><strong>sendmail-f</strong> is only for servers which require the use of the -f parameter to use sendmail. This is a security setting often used to prevent spoofing. Will cause errors if your host mailserver is not configured to use it.<br><strong>Qmail</strong> is used for linux/unix hosts running Qmail as sendmail wrapper at /var/qmail/bin/sendmail.<br><br>MOST SITES WILL USE [smtpauth].','12', '1', 'zen_cfg_select_option(array(\'PHP\', \'sendmail\', \'sendmail-f\', \'smtp\', \'smtpauth\', \'Gmail\',\'Qmail\'),', now());
