The Disable Gutenberg plugin provides you many other features like Enable Gutenberg for particular category, tag, or post type. But if you don't want any of the other options and simply want to disable the Gutenberg Editor then add the following action filters on functions.php file.
add_filter('gutenberg_can_edit_post', '__return_false', 5);
add_filter('use_block_editor_for_post', '__return_false', 5);
The gutenberg_can_edit_post filter is used WordPress < 5.0 beta and use_block_editor_for_post is used in WordPress >= 5.0.
