Modify Info Window Contents for Posts Using Hook

gistfile1.txt

add_filter('wpgmp_infowindow_post_message', 'wpgmp_infowindow_post_message',1,2 );

function wpgmp_infowindow_post_message($message,$map) {
	
  	//This will apply for map id 1, by accessing the map-object.
	if( $map->map_id == '1') {
	
		$message = "<h1>{marker_title}</h1>";
		
  	}  	

	return $message;

}

Changed post-object (representing current page or post) to map-object.