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If you extract the driver installer (--extract), the Linux 2.6.18 kernel patch is in ~/fglrx-install/packages/Fedora/ (or SuSE or the other patched distros). The patch is called kernel-2.6.18.diff. This file patches firegl_public.c.
Unfortunately, I haven't tried Gentoo in quite a while with ATI, so I am not sure off hand on the best approach for you with installing the fglrx drivers (other than possibly speaking with the ati-driver emerge maintainer about having the patch included).