mwatu
It requires osTicket v1.18.1 (or v1.17.5), requires the latest build of the OAuth2 plugin (for 1.18.x or 1.17.x) to be installed, and the web server must have the URL Rewrite Module installed and enabled for the site. Here is the relevant documentation to add and configure a system email. Here is the relevant documentation to configure OAuth2 Authorization for a Gmail email. Here is the relevant documentation to enable mail fetching and configuring a Cron Job (Linux) or Scheduled Task (windows).
You can find the IMAP and SMTP information for Gmail online.
Cheers.