Removing Gutenberg in WordPress Twenty Nineteen Theme is very simple as it is. Just need to add simple line code in theme functions.php file (I recommend to use child theme) to deactivate it.
What is GutenBerg?
Gutenberg is more than an editor. While the editor is the focus right now, the project will ultimately impact the entire publishing experience including customization (the next focus area).
Check this below video on how to remove Gutenberg in WordPress 2019 Theme.
Get the code at WordPress Code Snippet