glade.xml

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <!-- interface-requires gtk+ 3.0 -->
  <object class="GtkWindow" id="other_window">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkBox" id="box1">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkToolbar" id="toolbar2">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <child>
              <object class="GtkToolButton" id="test button">
                <property name="use_action_appearance">False</property>
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="use_action_appearance">False</property>
                <property name="is_important">True</property>
                <property name="label" translatable="yes">It's a star!</property>
                <property name="use_underline">True</property>
                <property name="stock_id">gtk-about</property>
              </object>
              <packing>
                <property name="expand">False</property>
                <property name="homogeneous">True</property>
              </packing>
            </child>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton" id="button3">
            <property name="label" translatable="yes">Normal gtk button in the window</property>
            <property name="use_action_appearance">False</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <property name="use_action_appearance">False</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
  </object>
</interface>

le picture

http://ubuntuone.com/4prgktamZNY5aK3o1Cm6od

program.lua

local lgi = require 'lgi'

local Gtk = lgi.Gtk

local builder = Gtk.Builder()
assert(builder:add_from_file('glade.xml'))
local ui = builder.objects

ui.other_window:show_all()

Gtk.main()