It's too bad that a browser extension is required to make this work. I've asked previously for GitLab to provide the necessary hooks so that any plugin could offer this type of functionality in a first class way, and was told that the proposal lacked a "concrete first need". I suppose SourceGraph has now demonstrated the usefulness of such a thing, which is great!
Ref: https://gitlab.com/gitlab-org/gitlab-ce/issues/33047