Disable Gutenberg Without Using Plugin

Disable Gutenberg Completely

// disable for posts
add_filter('use_block_editor_for_post', '__return_false', 10);

// disable for post types
add_filter('use_block_editor_for_post_type', '__return_false', 10);

Older versions of WP/Gutenberg

// disable for posts
add_filter('gutenberg_can_edit_post', '__return_false', 10);

// disable for post types
add_filter('gutenberg_can_edit_post_type', '__return_false', 10);

Conditional technique

// Disable Gutenberg
if (version_compare($GLOBALS['wp_version'], '5.0-beta', '>')) {	
	// WP > 5 beta
	add_filter('use_block_editor_for_post_type', '__return_false', 10);	
} else {	
	// WP < 5 beta
	add_filter('gutenberg_can_edit_post_type', '__return_false', 10);	
}

Disable Gutenberg for Custom Post Types

function wc_disable_gutenberg($is_enabled, $post_type) {

	if ($post_type === 'book') return false; // change book to your post type

	return $is_enabled;

}
add_filter('use_block_editor_for_post_type', 'wc_disable_gutenberg', 10, 2);