functions.php

<?php

/**
 * Gutenberg layout class
 * @see https://www.billerickson.net/change-gutenberg-content-width-to-match-layout/
 *
 * @param string $classes
 * @return string
 */
function ea_gutenberg_layout_class( $classes ) {
	$screen = get_current_screen();
	if( ! $screen->is_block_editor() )
		return $classes;

	$layout = false;
	$post_id = isset( $_GET['post'] ) ? intval( $_GET['post'] ) : false;
  
	// Get post-specific layout
	if( $post_id )
		$layout = genesis_get_custom_field( '_genesis_layout', $post_id );
    
	// Pages use full width as default, see below
	if( empty( $layout ) && 'page' === get_post_type() )
		$layout = 'full-width-content';
    
	// If no post-specific layout, use site-wide default
	elseif( empty( $layout ) )
		$layout = genesis_get_option( 'site_layout' );

	$classes .= ' ' . $layout . ' ';
	return $classes;
}
add_filter( 'admin_body_class', 'ea_gutenberg_layout_class' );

/**
 * Full width layout for pages as default
 *
 * @param string $layout 
 * @return string
 */
function ea_full_width_pages( $layout ) {
	if( is_page() )
		$layout = 'full-width-content';
	return $layout;
}
add_filter( 'genesis_pre_get_option_site_layout', 'ea_full_width_pages' );