I've spoken with a company that provides an app that they are (they might comment below) interested in integrating / making work with GitLab Pages.
As far as I know, there is no API now to provision pages, set a domain name and add a certificate. This company is interested in automating those parts.
API endpoints for Pages domain, certificate, so that external parties can integrate with GitLab Pages.
Links / references
@markpundsack @ayufan has this been on your mind? Is it possible on the short term?