Put the add_filter() call in the init function:
public function init(){
parent::init();
add_filter("gform_submit_button", array($this, "form_submit_button"), 10, 2);
add_filter("gform_confirmation", array( $this, 'custom_confirmation' ), 10, 4 );
}
And then add your custom_confirmation() function as a method to the class:
class GFSimpleAddOn extends GFAddOn {
/* all the other properties and methods */
public function custom_confirmation($confirmation, $form, $lead, $ajax){
$gfe = $lead['id'];
// rest of your code
}
}