via plugin: https://wordpress.org/plugins/disable-gutenberg/ tutorial: https://digwp.com/2018/04/how-to-disable-gutenberg/

functions.php

// 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);



//Disable Gutenberg for Custom Post Types

function disable_gutenberg_page($is_enabled, $post_type) {	
	if ($post_type === 'page') return false;	
	return $is_enabled;	
}
add_filter('use_block_editor_for_post_type', 'disable_gutenberg_page', 10, 2);

function disable_gutenberg_block($is_enabled, $post_type) {	
	if ($post_type === 'blocks') return false;	
	return $is_enabled;	
}
add_filter('use_block_editor_for_post_type', 'disable_gutenberg_block', 10, 2);

Works perfect, thanks a lot