GitLab doesn't try hard enough to get people to sign in/sign up
If I go to a commit page (e.g., https://gitlab.com/crussell/amVim/commit/b21fd8511fb7a0d73c7f056159ed9bbcacbae771) and I'm not logged in, it should tell me I can leave comments by signing in or creating an account. Emphasize that you can sign in using GitHub/other OAuth providers. (This should be mentioned on the same page, underneath the diff where the comment widgets usually are.)
If I go to a project page when not signed in, it should still show the widgets to create a new issue or fork the project, but redirect to the sign in/sign up page when clicked. (It does show the widget to star the project.)
If I go to a project's issues page and I'm not logged in, it should show the +New Issue widget, but redirect to the sign in page when clicked.
Finally, coming full circle, if I'm on the actual page for a single issue, it should have in-page text where the comment field would normally be, telling me to sign up or sign in to comment. (Currently, the thumbs up and thumbs down buttons are shown, but they do nothing. The smily face that attaches extra emojis is not shown.) Like the commit page, this should be an actual message in the page, somewhere around where the comment box would normally be, with an explicit message that you can sign in with the third-party providers.
Alternatively, if you want to go for bonus points, don't say anything about needing to sign in on an issue page. Treat it just like the previous couple of cases: show the comment box and the "Add Comment" button, but when I click it, it should show the sign up/sign in buttons either inline, or by redirecting to a page that will post the comment upon successful completion (including the ones for third-party providers). This is trickier, because it requires temporarily storing unposted comments on the server, but they can be automatically pruned after some reasonable delay.