The site is not accessible to an ipv6-only client. I'm not sure if this is the case from a system configured to be only ipv6, but my isp lost dhcp this morning so I was stuck without an ipv4 address. While mapping what was available to me, I noticed that pretty much every Linux forum I'm a member of was inaccessible, while mainstream sites like facebook, mainstream news and anything related to Google were available.
Not sure if this is correct, but it may be as simple as adding AAAA records. The IPV6 version of forums.gentoo.org is ::204.187.15.12, or ::CCBB:0F0C
It's not as simple as that. One can't "convert" IPv4 addresses into IPv6 ones. The IPv4 and IPv6 have completely separate address spaces. You can still map IPv4 addresses to IPv6 ones, but that's mostly so you can only support IPv6 addresses in your application. That said, the forum's host seems to use an IPv6-capable host, so this should still be a fairly simple fix.
Re-assigning since forum-mods@ can not do anything about this at present time.