> In other words, if Gentoo ships Gnome3, then these .typelib files must have
> been packaged and installed somewhere.
> Unfortunately I don't know Gentoo well enough to tell you how to look for them.
Gentoo is a meta distribution, meaning it provides mechanisms and tools to manage dependencies and a good QA scheme to ensure stability, but packages are compiled on your own systems based on your own requirement. The big added value is you got a system optimized for your hardware and containing only what you're looking for : a big boost compared to other distributions on my old hardware (it's the same with NetBSD's pkgsrc for
example).
The drawback is you got only what you requested, and in my case, I never care about introspection till now as I didn't have any need for it.
I checked on my system where I use LXDE : I got some .typelib files
nunux laurent # find /usr -name
'*.typelib'
/usr/lib/girepository-1.0/GObject-2.0.typelib
/usr/lib/girepository-1.0/xfixes-4.0.typelib
/usr/lib/girepository-1.0/freetype2-2.0.typelib
/usr/lib/girepository-1.0/cairo-1.0.typelib
/usr/lib/girepository-1.0/GIRepository-2.0.typelib
/usr/lib/girepository-1.0/xrandr-1.3.typelib
/usr/lib/girepository-1.0/xft-2.0.typelib
/usr/lib/girepository-1.0/Gio-2.0.typelib
/usr/lib/girepository-1.0/libxml2-2.0.typelib
/usr/lib/girepository-1.0/fontconfig-2.0.typelib
/usr/lib/girepository-1.0/xlib-2.0.typelib
/usr/lib/girepository-1.0/GLib-2.0.typelib
/usr/lib/girepository-1.0/GL-1.0.typelib
/usr/lib/girepository-1.0/DBusGLib-1.0.typelib
/usr/lib/girepository-1.0/DBus-1.0.typelib
/usr/lib/girepository-1.0/GModule-2.0.typelib
/usr/lib/girepository-1.0/GUdev-1.0.typelib
and I guess it's only because I included some gnomeish tools. Unfortunately, no GTK, so I think I'll have to reinstall it
...
