You have missed out the signal names in the gui.glade file : See the two tags called signal below.
Reference : http://python-gtk-3-tutorial.readthedocs.org/en/latest/builder.html
<?xml version="1.0" encoding="UTF-8"?>
<interface>
<!-- interface-requires gtk+ 3.0 -->
<object class="GtkWindow" id="window1">
<property name="can_focus">False</property>
<signal name="delete-event" handler="onDeleteWindow" swapped="no"/>
<child>
<object class="GtkBox" id="box1">
<property name="visible">True</property>
<property name="can_focus">False</property>
<property name="orientation">vertical</property>
<property name="spacing">4</property>
<child>
<object class="GtkButton" id="button1">
<property name="label" translatable="yes">Install</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>
<signal name="pressed" handler="onButtonPressed" swapped="no"/>
</object>
</child>
</object>
</child>
</object>
</interface>