Add Gutenberg suport and a block template to an existing portfolio post type (e.g. from the Portfolio Post Type or Genesis Portfolio Pro plugin)

portfolio-block-template.php

<?php
/**
 * Adds block template to existing post type 'portfolio' with taxonomies 'portfolio_category' and 'portfolio_tag'.
 */

// Add Block Template to existing portfolio post type
function bbp_add_portfolio_template() {
    $post_type_object = get_post_type_object( 'portfolio' );
    $post_type_object->show_in_rest = true;
    $post_type_object->rest_base = 'portfolio';
    $post_type_object->template = array(
		array( 'core/heading', array(
			'placeholder' => 'Executive Summary',
			'level' => '2',
			) 
		),
		array( 'core/paragraph', array(
			'placeholder' => 'Put the problem and the results you got into about 160 characters, with keywords. Then you can use it for your meta description.',
			'customFontSize' => '24',
			) 
		),
		array( 'core/columns', 
			array(
				'align' => 'wide',
				'columns' => '2',
			), 
		array(
			array( 'core/column', array(), array(
				array( 'core/image', array() ),
			) ),
			array( 'core/column', array(), array(
				array( 'core/heading', array(
					'placeholder'=> 'Client Company Name',
					'level'	=> '3',
				) ),
				array( 'core/paragraph', array(
					'placeholder' => 'Describe your client: industry, company size, who their customer is, what they do.',
				) ),
			) ),
		) ),
		array('core/quote', array(
		) ),
		array( 'core/heading', array(
			'placeholder' => 'The Challenge',
			'level' => '2',
		) ),
		array( 'core/paragraph', array(
			'placeholder' => 'Why did your client need your services?',
		) ),
		array( 'core/image', array(
			'align' => 'wide',
		) ),
		array( 'core/heading', array(
			'placeholder' => 'Your Solution',
			'level' => '2',
		) ),
		array( 'core/paragraph', array(
			'placeholder' => 'Write two or three short paragraphs describing what you did to solve the problem.',
		) ),
		array( 'core/image', array(
			'align' => 'wide',
		) ),
		array( 'core/heading', array(
			'placeholder' => 'Results and ROI',
			'level' => '2',
		) ),
		array( 'core/paragraph', array(
			'placeholder' => 'You need a keyword-rich sound bite with numbers in it, e.g. tripled revenue or cut production time in half.',
			'customFontSize' => '20',
			'customTextColor' => '#FF6900',

		) ),
		array( 'core/paragraph', array(
			'placeholder' => 'Provide more detail about how happy the client is and why. You could put another quote here instead.',
		) ),
		array( 'core/image', array(
			'align' => 'wide',
		) ),
		array( 'core/paragraph', array(
			'placeholder' => 'Write your call to action here. Give it a large font size and a color background.',
			'customFontSize' => '28',
			'customTextColor' => '#ffffff',
			'customBackgroundColor' => '#313131',
		) ),
		array( 'core/button', array(
			'customBackgroundColor' => '#CF2E2E',
			'customTextColor' => '#ffffff',
			'align' => 'center',
		) ),
	);
}
add_action( 'init', 'bbp_add_portfolio_template' );

// Add REST API support for portfolio categories
function add_portfolio_cat_to_api() { 
	$mytax = get_taxonomy( 'portfolio_category' ); 
	$mytax->show_in_rest = true; 
} 
add_action( 'init', 'add_portfolio_cat_to_api', 30 );


// Add REST API support for portfolio tags
function add_portfolio_tag_to_api() { 
	$mytax = get_taxonomy( 'portfolio_tag' ); 
	$mytax->show_in_rest = true; 
} 
add_action( 'init', 'add_portfolio_tag_to_api', 30 );