Analogously to bug 478050 there is a gdk-pixbuf loader supporting webp. It allows webp support in many GKT applications like media-gfx/geeqie or media-gfx/mcomix. The code is located at https://github.com/aruiz/webp-pixbuf-loader I built it without problems and it works as expected (at least with geeqie and mcomix). It should probably be pulled in by a USE flag in x11-libs/gdk-pixbuf.
I just created an ebuild for this package and submitted a pull request for it to gentoos github: https://github.com/gentoo/gentoo/pull/2076
commit 546c66647a2c926ed94c72582e1eb7c640f4eaa8 (HEAD -> master, origin/master, origin/HEAD) Author: Andy Kittner <andy.kittner@gmail.com> AuthorDate: Fri Aug 12 22:45:12 2016 +0200 Commit: Patrice Clement <monsieurp@gentoo.org> CommitDate: Sat Aug 27 16:05:45 2016 +0200 x11-libs/gdk-pixbuf-loader-webp: new package. A format plugin for gdk-pixbuf that allows GTK applications (e.g media-gfx/geeqie) to read webp images. Gentoo-Bug: https://bugs.gentoo.org/575030 Package-Manager: portage-2.3.0 Closes: https://github.com/gentoo/gentoo/pull/2076 Signed-off-by: Patrice Clement <monsieurp@gentoo.org> .../gdk-pixbuf-loader-webp-20160328234507.ebuild | 39 ++++++++++++++++++++++ x11-libs/gdk-pixbuf-loader-webp/metadata.xml | 19 +++++++++++ 2 files changed, 58 insertions(+) create mode 100644 x11-libs/gdk-pixbuf-loader-webp/gdk-pixbuf-loader-webp-20160328234507.ebuild create mode 100644 x11-libs/gdk-pixbuf-loader-webp/metadata.xml Commited to the tree. Thanks!