Gimp 2.9.6 added a hard dependency on glib-networking with SSL suppport (https://git.gnome.org/browse/gimp/commit/?id=bb196607693b2a8fa95f033966506cbf3c377d81) but this dependency is not reflected in the ebuild. If glib-networking is not installed, or is built with -ssl, gimp will fail to build.
Created attachment 490984 [details] emerge build log
Created attachment 490986 [details] gimp configure log
For what it's worth, using --without-appdata-test doesn't make the dependency optional despite what build.log is hinting. It's disappointing that an image editor requires network access.
Excellent report, thanks! Should be fixed now: commit 897e4677099a157a132f76bb4252a7ea5cd7bd04 Author: Sebastian Pipping <sping@g.o> Date: Tue Aug 29 21:25:28 2017 +0200 media-gfx/gimp: Add missing dependency (bug #629168) Thanks to Daniel Harding! Package-Manager: Portage-2.3.8, Repoman-2.3.3 media-gfx/gimp/gimp-2.9.6.ebuild | 1 + media-gfx/gimp/gimp-9999.ebuild | 1 + 2 files changed, 2 insertions(+) https://github.com/gentoo/gentoo/commit/897e4677099a157a132f76bb4252a7ea5cd7bd04