gistfile1.php

<?php
// Page.php
class Page extends SiteTree {

  public static $many_many = array(
  	'TestObjects'		=>		'TestObject'
	);

	public static $many_many_extraFields = array(
		'TestObjects'		=>		array(
			'ExtraDecimal'		=>		'Decimal',
			)
	);

	public function getCMSFields() {
		$fields = parent::getCMSFieldS();

		$grid = new GridField(
		    'ExampleGrid',
		    'Example Grid',
		    $this->TestObjects(),
		    GridFieldConfig::create()
		        ->addComponent(new GridFieldButtonRow('before'))
		        ->addComponent(new GridFieldToolbarHeader())
		        ->addComponent(new GridFieldTitleHeader())
		        ->addComponent(new GridFieldEditableColumns())
		        ->addComponent(new GridFieldDeleteAction())
		        ->addComponent(new GridFieldAddNewInlineButton())
		);

		$fields->addFieldToTab('Root.Main', $grid);
		return $fields;
	}

}

// TestObject.php
class TestObject extends DataObject {
  public static $db = array(
		'RegularDecimal'		=>		'Decimal'
	);
	public static $belongs_many_many = array(
		'Pages'					=>		'Page'
	);
	public static $summary_fields = array(
		'RegularDecimal', 'ExtraDecimal'
	);
}