gitlab seem to have changed both their documentation and their web UI, so the above guidelines no longer apply. I've found the most recent info at https://old.reddit.com/r/gnome/comments/1ebc8aw/notifications_toggle_for_issue_on_gnomeshell/. For muting notifications for a particular issue, click the vertically arranged triple-dot icon located to the right of the issue title, and in the menu that pops up, disable Notifications.