There's an ebuild in the Gentoo Bugzilla if you know how to 'roll your own'. (If you don't, read my SL Forum thread on how to build gscan2pdf for an example of using an ebuild from the Gentoo Bugzilla in a local overlay - just use SL Forum search facility with the keyword "gscan2pdf").
EDIT: Tip for the future: google keywords "gentoo bugzilla" and then type in the name of the application/package you are looking for into the Gentoo Bugzilla search box. In this case "frets on fire" will get you
http://bugs.gentoo.org/show_bug.cgi?id=143388