There is a propiertary driver iscan-plugin-gt-1500_2.1.1-1_amd64.deb from http://linux.avasys.jp/drivers/scanner-plugins/ and no ebuild to use it see also http://bugs.gentoo.org/show_bug.cgi?id=277117
We would never stop filing bug reports if we'd want a separate one for each driver. *** This bug has been marked as a duplicate of bug 277117 ***