I want to center my windows title like so:

Instead of having them aligned to the right:

Is there a setting I can change in gconf-editor or is this something controlled by each theme?
This is indeed part of the theme. It is not configurable through gconf-editor. Here is an example of how to edit a theme's XML file.
Before:
<draw_ops name="title-text-focused">
<title color="shade/#000000/0.8" x="0" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="1" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="2" y="(((height - title_height) / 2)+1)"/>
<title color="#f0f0f0" x="0" y="(height - title_height) / 2"/>
</draw_ops>
<draw_ops name="title-text-unfocused">
<title color="shade/#000000/0.8" x="0" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="1" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="2" y="(((height - title_height) / 2)+1)"/>
<title color="#808080" x="0" y="(height - title_height) / 2"/>
</draw_ops>
After:
<draw_ops name="title-text-focused">
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="#f0f0f0" x="((3 `max` (width-title_width)) / 2)" y="(height - title_height) / 2"/>
</draw_ops>
<draw_ops name="title-text-unfocused">
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="shade/#000000/0.8" x="((3 `max` (width-title_width)) / 2)" y="(((height - title_height) / 2)+1)"/>
<title color="#808080" x="((3 `max` (width-title_width)) / 2)" y="(height - title_height) / 2"/>
</draw_ops>