I want to change the default template for my pages to a template called fullwidthpage.php.

I have seen this question posted and answered for pre-Gutenberg WordPress, but I have not found a working answer for Gutenberg WordPress (version 5.3.2 at the time of this question).

This is the non-working answer I found. When I try the non-working answer the template is set to fullwidthpage.php but when I try to update the page I get a message that says “Updating failed.”