Adding a Custom Category to Gutenberg

gutenberg-custom-category.php

<?php

  add_filter( 'block_categories', function( $categories, $post ) {
      return array_merge(
          $categories,
          [
              [
                  'slug' => 'my-category',
                  'title' => __( 'My Custom Category', 'my-namespace' ),
              ],
          ]
      );
  }, 10, 2 );