Disable Gutenberg except for “Frontpage” page - found via: https://codeshare.io/29Kj8K

gutenberg-frontpage-only.php

<?php

/**
 * Disable Gutenberg by Default
 * but let others easily override that by changing hook priority to 5
 * https://justnorris.com/how-to-dynamically-enable-gutenberg/
 */
add_filter( 'gutenberg_can_edit_post', '__return_false', 5 );
add_filter( 'use_block_editor_for_post', '__return_false', 5 );


// But enable Gutenberg in a page with title “Frontpage”

function maybe_load_gutenberg( $can_edit, $post ) {
  $mypage = get_page_by_title( 'Frontpage' );

  // Enable Gutenberg
  if ( $mypage->ID === $post->ID ) {
		return true;
	}

	return $can_edit;
}

// Gutenberg >= 3.5
add_filter( 'gutenberg_can_edit_post', 'maybe_load_gutenberg', 10, 2 );

// WordPress >= 5.0
add_filter( 'use_block_editor_for_post', 'maybe_load_gutenberg', 10, 2 );