Although the gentoo kernel sources contain on option for iprange, there is no way to get its module actually compiled so that it can be used in iptables. I think the ebuild for iptables should be altered to allow the use of iprange. See http://forums.gentoo.org/viewtopic.php?t=64136&highlight=iprange for more information.
The iptables ebuild is not maintained by the x86-kernel team. Booting this back to bug-wranglers since it seems there is no clear maintainer of the ebuild.
Seems to get built for me with 1.2.9: >>> ... >>> /lib/iptables/libipt_iprange.so >>> ...