The gv-extends tag is used to define which master layout a page uses when being executed by Governor. Only one extend tag can be used per page template. If a page does not define what layout it extends, the default layout of master.html is used.
<!-- gv-extends defines an alternative master file, in this case '404-master.html', instead of using the default master.html -->
Need an alternative navigation just for your blogs section? Use gv-extends to change large, mostly static elements such as headers or footers.