Installed add-on "Atlassian Cloud for Gmail." When I open an existing message in Gmail (Chrome, Win10), I am able to open right sidebar and create a Jira issue, but cannot look up an existing issue and associate it with that message. Could not find an option within the Jira task to link to an email message. Seems important but missing. Comments welcome.
p.s. = Shortly after posting, I did learn how to forward said email message to Jira and have it show up as a comment in the existing issue. One must add the issue ID to the subject line (or anywhere in the body?), and one must know the address of the Jira default mail handler to which it is to be sent. Still curious if this can done inside Gmail with the add-on "Atlassian Cloud for Gmail." Does not seem to be offered. Maybe because the authors assume that everyone knows how to forward email to Jira. I did not, and my search of the Documentation and Community resources did not produce relevant hits.