GitLab should make it so that anyone who reaches a 4xx or 5xx error page should be able to trivially (as in one click from the error page) file a bug about it on the GitLab tracker. Make sure to pre-populate the template with URL, status code, and referer.
You're assuming they're not doing that automatically in the background. I'd rather assume that they're already getting at least all 5xx reports. (it would be a big failure otherwise)