If you’re getting an email about an issue, it’s because you’ve interacted with it in the past, probably because you’ve liked it, commented on it, or created it in the first place. You also may have ticked on the “notifications” option for the issue.
As for what to do with the email, are you interested in getting further email notifications about this issue?
If you want to keep getting notifications about that issue, absorb the information in the email and then do whatever you do with emails you don’t need anymore.
If you don’t want to be notified about changes to the issue anymore, click the “unsubscribe” link in the email.
If you don’t want to get any gitlab emails ever again, there’s a link for that, too.
Whatever you do, avoid replying to the email unless you have something meaningful to add, because that will add a publicly-viewable comment to the issue and notify everyone else who’s following that issue.