document how to use GitLab through email
(Note: this is just for issues right now as I've not been dealing with merge requests lately. But a bunch of the notes below should apply to them, too)
@gk started writing this and I'm moving it into an issue.
Creating a new issue
Clicking on the per project issues gives a link at the bottom of the page, say "Email a new issue to this project".
That link should go into the From of your email. The subject is the title of the issue and the body the description. You can start right away using shortcuts in the body, like /assign @foo, /estimate 1d etc. (see: https://docs.gitlab.com/ee/user/project/issues/managing_issues.html#new-issue-via-email for more details)
Adding a comment to an existing issue
Replying to an existing comment
You need to have notifications enabled for this part and then just reply to the particular comment as you would reply to an email in a thread (see: https://docs.gitlab.com/ee/administration/reply_by_email.html for more details)
Creating a new comment
This is not easily doable right now (see: https://gitlab.com/gitlab-org/gitlab/-/issues/18816). However, it works if you have notifications enabled and then reply to any notification email for the issue of interest by replacing everything that would get quoted with the comment you want to add. This works as well with shortcuts like /estimate 1d or /spend -1h (note: for those you won't get notification emails back, though, while for others like /assign @foo you would).
Using quick actions to update an issue
There are a bunch of quick actions available which are handy to update an issue (see: https://gitlab.torproject.org/help/user/project/quick_actions.md). As mentioned above they can be sent by email as well, both within a comment (be it as a reply to a previous one or in a new one) or just instead of it. So, if you for example want to update the amount of time spent on ticket $foo by one hours, find any notification email for that issue and reply to it by replacing any quoted text with "/spend 1h".