The first implementation for the verification of a custom domain in Pages (https://gitlab.com/gitlab-org/gitlab-ce/issues/29497) uses DNS. We can consider additional verification methods for users that cannot set up a TXT entry to validate the domain. A possible solution is a verification code sent to an email address that is linked to the owner of the domain (whois). Another option is to improve dns validation with additional resources, like `CNAME` entries.