Gnome Subtitles is in Portage. Emerge it as usual with:
You may need to unmask it before, to use the latest version, with:
echo "media-video/gnome-subtitles" >> /etc/portage/package.keywords
Use the icon in the Gnome Menu or run it from a terminal with:
If the latest version isn't in Portage yet, you can usually find it in Gentoo Bugzilla.