gallery.php

<?php
namespace Grav\Plugin;

use Grav\Common\Plugin;
use RocketTheme\Toolbox\Event\Event;

class GalleryPlugin extends Plugin
{
    public static function getSubscribedEvents()
    {
        return [
            'onTwigExtensions' => ['onTwigExtensions', 0]
        ];
    }

    /**
     * Initialize the plugin
     */
    public function onTwigExtensions()
    {
        require_once(__DIR__ . '/GalleryTwigExtension.php');
        $this->grav['twig']->twig->addExtension(new GalleryTwigExtension()); 
    }

}