fields.php

add_action( 'cmb2_init', 'show_on_page_template' );
/**
 * Hook in and add a metabox to demonstrate repeatable grouped fields
 */
function show_on_page_template() {

	$cmb = new_cmb2_box( array(
			'id'           => 'contact-information',
			'title'        => 'Contact Information',
			'show_on' => array( 'key' => 'page-template', 'value' => 'page-nosidebar.php' ),
			'context'      => 'normal', //  'normal', 'advanced', or 'side'
			'priority'     => 'high',  //  'high', 'core', 'default' or 'low'
			'show_names'   => true, // Show field names on the left
	) );

	$cmb->add_field( array(
		'name'         => 'Testing Field Parameters',
		'id'           => $prefix . 'parameters',
		'type'         => 'text',
		'before_row'   => 'yourprefix_before_row_if_2', // callback
		'before'       => '<p>Testing <b>"before"</b> parameter</p>',
		'before_field' => '<p>Testing <b>"before_field"</b> parameter</p>',
		'after_field'  => '<p>Testing <b>"after_field"</b> parameter</p>',
		'after'        => '<p>Testing <b>"after"</b> parameter</p>',
		'after_row'    => '<p>Testing <b>"after_row"</b> parameter</p>',
	) );


}

functions.php

/**
 * Metabox for Page Template
 * @author Kenneth White
 * @link https://github.com/WebDevStudios/CMB2/wiki/Adding-your-own-show_on-filters
 *
 * @param bool $display
 * @param array $cmb
 * @return bool display metabox
 */
function be_metabox_show_on_template( $display, $cmb ) {
    if ( ! isset( $cmb['show_on']['key'], $cmb['show_on']['value'] ) ) {
        return $display;
    }

    if ( 'page-nosidebar.php' !== $cmb['show_on']['key'] ) {
        return $display;
    }

    $post_id = 0;

    // If we're showing it based on ID, get the current ID
    if ( isset( $_GET['post'] ) ) {
        $post_id = $_GET['post'];
    } elseif ( isset( $_POST['post_ID'] ) ) {
        $post_id = $_POST['post_ID'];
    }

    if ( ! $post_id ) {
        return false;
    }

    $template_name = get_page_template_slug( $post_id );
    $template_name = ! empty( $template_name ) ? substr( $template_name, 0, -4 ) : '';

    // See if there's a match
    return in_array( $template_name, (array) $cmb['show_on']['value'] );
}
add_filter( 'cmb2_show_on', 'be_metabox_show_on_template', 10, 2 );