When using a custom domain for GitLab Pages (i.e. mysite.com), it seems that default domain (myaccount.gitlab.io/mysite) is also active, so content is published in both domains at once.
How can I disable default domain? If there is an option, I can’t find it.
I have looked into GitLab's documentation, forums and Googled it, but not found how to disable this behaviour.
Indeed you can't because of the nature of how customDomain is implemented. As stated in the doc https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/ the internal domain is a prerequisite even you use A handler with IP.