Error After Installing Gutenberg plugin (caused by attempt to stop WP from stripping out tags)

Download a copy of WordPress and extract the wp-admin and wp-includes folders over the top of your existing folders, replacing them.

Then, you’re left with the original problem to solve, which can be done by following the original blog post correctly. Filters and actions don’t go in the wp-hooks.php file, they go in a plugin or a themes functions.php