A few months ago the following issue was fixed in ethtool: https://patchwork.ozlabs.org/patch/806563/ "Cannot get device udp-fragmentation-offload settings: Operation not supported" Renaming ethtool-4.8.ebuild to ethtool-4.13.ebuild is working fine for me.
Actually we have that version in portage :) https://github.com/gentoo/gentoo/blob/master/sys-apps/ethtool/ethtool-4.13.ebuild
It was probably too late, sorry! °°