There is a known problem importing the Gtk library from seed on
systems where seed is built against Gtk2, if Gtk3 is also installed;
this is because both Gtk versions install gobject-introspection
typelib files in the repository, and unless the desired version is
explicitly specified.

Simply importing Gtk fails as such:

> imports.gi.Gtk
seed: symbol lookup error: /usr/lib64/libgtk-x11-3.0.so.0: undefined
symbol: g_application_get_type

The workaround is to specify the version as such:

> // note the quotes; otherwise 2.0 gets evaluated as just 2
> imports.gi.versions.Gtk = "2.0";
2.0
> Gtk = imports.gi.Gtk;
[object Object]
> 

See example 3 on the Reference Manual's Importer section:
http://library.gnome.org/devel/seed/stable/seed-importer.html