The "network" items in a Kickstart file only allow one default gateway, which is insufficient for typical dual-stack IPv4 and IPv6 installations. The files in network-scripts already support separate IPv4 and IPv6 gateway configuration for each interface; Kickstart should support that. A workaround is to append either the IPv4 or IPv6 gateway in the post-installation script section.
network --ipv6gateway option has been added for F20.