add_options_menu.php

<?php
add_action( 'admin_menu', 'register_my_settings_page' );

function register_my_settings_page() {
    add_submenu_page( 'edit.php?post_type=gallery', 'Media Selector', 'Media Selector', 'manage_options', 'gallery-settings', 'media_selector_settings_page_callback' );
//add_submenu_page( '<PARENT SLUG>', '<PAGE TITLE>', '<MENU TITLE>', '<ROLE>', '<MENU SLUG>', '<CALLBACK NAME>' );
}

function media_selector_settings_page_callback() {

    ?>
    <div class="wrap">
        <h2>Your Plugin Name</h2>

        <form method="post" action="options.php">
            <?php settings_fields( 'gallery-settings' ); ?>
            
            <input type="text" name="new_option_name" value="<?php echo get_option('new_option_name'); ?>" />
            <?php ?>
            <p class="submit">
                <input type="submit" class="button-primary" value="<?php _e('Save Changes') ?>" />
            </p>

        </form>
    </div>
    <?php
}

add_action( 'admin_init', 'register_my_settings' );

function register_my_settings () {
    register_setting( 'gallery-settings', 'new_option_name' );
}