The DESCRIPTION in the glsa-check manpage has a link http://www.gentoo.org/security , but this link does not exist. The correct link is probably https://www.gentoo.org/support/security/ .
glsa-check is included with >=sys-apps/portage-2.3.72 (bug 463952).
This has more specific information: https://www.gentoo.org/support/security/vulnerability-treatment-policy.html
This link has been removed with the latest man page updates.