magento-test/app/code/Magento/MediaGalleryCmsUi/etc/adminhtml/routes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
    <router id="admin">
        <route id="media_gallery_cms" frontName="media_gallery_cms">
            <module name="Magento_MediaGalleryCmsUi" before="Magento_Backend" />
        </route>
    </router>
</config>