How to configure it?

The element attribute identifier name the snippet-based rendering engine (SBRE) must use to identify elements is directly specified in the main template file, by means of a processing instruction named gwc-id. This instruction must be located at the root level of the document.

Example:

<?xml version="1.0" encoding="utf-8"?>
<?gwc-id x:Name?>
<html xmlns="http://www.w3.org/1999/xhtml"
  xmlns:x="http://www.server.com/ns/xxx"
  xmlns:gwc="http://www.4js.com/GWC">
  ...

Note:

The gwc-id processing instruction is optional. The default element identifier attribute name is id.
If it belongs to a namespace, the attribute name must be prefixed by a namespace alias that is declared on the root element. In this example, the Name attribute belongs to the http://www.server.com/ns/xxx namespace.