$ emerge dev-python/pygobject x11-libs/gtk+:3 -pv

These are the packages that would be merged, in order:

Calculating dependencies  ... ... done!
[ebuild   R    ] x11-libs/gdk-pixbuf-2.36.12:2::gentoo  USE="X introspection* -debug -jpeg -jpeg2k {-test} -tiff" ABI_X86="32 (64) (-x32)" 0 KiB
[ebuild   R    ] dev-libs/atk-2.24.0::gentoo  USE="introspection* -nls {-test}" ABI_X86="32 (64) (-x32)" 0 KiB
[ebuild   R    ] x11-libs/pango-1.42.4::gentoo  USE="X introspection* {-test}" ABI_X86="32 (64) (-x32)" 0 KiB
[ebuild   R    ] dev-python/pygobject-3.24.1:3::gentoo  USE="cairo threads -examples {-test}" PYTHON_TARGETS="python2_7 python3_5 python3_6 -python3_4" 0 KiB
[ebuild   R    ] x11-libs/gtk+-3.22.30:3::gentoo  USE="X introspection* (-aqua) -broadway -cloudprint -colord -cups -examples {-test} -vim-syntax -wayland -xinerama" ABI_X86="32 (64) (-x32)" 0 KiB

Total: 5 packages (5 reinstalls), Size of downloads: 0 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by x11-libs/gtk+-3.22.30::gentoo
# required by x11-libs/gtk+:3 (argument)
>=x11-libs/gdk-pixbuf-2.36.12 introspection
# required by x11-libs/gtk+-3.22.30::gentoo
# required by x11-libs/gtk+:3 (argument)
>=x11-libs/pango-1.42.4 introspection
# required by x11-libs/gtk+-3.22.30::gentoo
# required by x11-libs/gtk+:3 (argument)
>=dev-libs/atk-2.24.0 introspection