Magento – Module created gives Error filtering template: {Classname} does not implement BlockInterface

I need a way to show products from a category and created a module following the advice of this answer of Marius (https://magento.stackexchange.com/users/146/marius) here: Magento 2: get product collection using category id

I did create a new module though with these files:

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    '{Company}_GetProductCollection',
    __DIR__
);

as registration.php in app/code/{Company}/GetProductCollection

Then

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="{Company}_GetProductCollection" setup_version="1.0.0"></module>
</config>

as module.xml in in app/code/{Company}/GetProductCollection/etc and

<?php

namespace {Company}\GetProductCollection\Block;

class CeloGetProductCollection
{

  protected $categoryFactory;

  public function __construct(

      \Magento\Catalog\Model\CategoryFactory $categoryFactory
  ){

      $this->categoryFactory = $categoryFactory;

  }

  public function getCategory()
  {
    $categoryId = $this->getCategoryId();
    $category = $this->categoryFactory->create()->load($categoryId);
    return $category;
  }

  public function getProductCollection()
  {
    return $this->getCagetory()->getProductCollection()->addAttributeToSelect('*');
  }

}

as CeloGetProductCollection.php in app/code/{Company}/GetProductCollection/Block

I registered the module successfully with setup:upgrade.

Then bound the file into the page Home Page with:

{{block class="{Company}\GetProductCollection\Block\CeloGetProductCollection" category_id="15" template="Magento_Theme::html/discover_news.phtml"}}

and discover_news.phtml is in my theme directory. Having this content:

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
?>
<?php foreach ($this->getProductCollection() as $product) : ?>
    print_r($product,true);
<?php endforeach;?>

What's missing? Because I get the page but instead the oroduct info I get:

Module created gives Error filtering template:
{Company}\GetProductCollection\Block\CeloGetProductCollection does not
implement BlockInterface

Anyone know what's wrong?