The ‘Add to calendar’ button may be greyed out due to a few conditions:

If the ticket is in a Closed status

• Change the ticket status to anything other than Closed E.g Open or On Hold

If the time zone of the user (in Gorelo) is different than their computer/browser

• Navigate to Settings → General → Users → edit the user and make sure their Time Zone is the same as what’s on their computer/browser.