How to call CMS page using controller in Magento 2

There is already an answer for this, Refer to link Link to Click. But I think you want to create a controller a load page using controller route id. So you have to do something like which I did below-

Create controller like-

<?php
/**
 * Copyright ©  All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace Gag\Front\Controller\Index;

class Index extends \Magento\Framework\App\Action\Action
{

    protected $resultPageFactory;

    /**
     * Constructor
     *
     * @param \Magento\Framework\App\Action\Context  $context
     * @param \Magento\Framework\View\Result\PageFactory $resultPageFactory
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory
    ) {
        $this->resultPageFactory = $resultPageFactory;
        parent::__construct($context);
    }

    /**
     * Execute view action
     *
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {
        return $this->resultPageFactory->create();
    }
}

Create a routes.xml like-

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
    <router id="standard">
        <route id="frontcms" frontName="frontcms">
            <module name="Gag_Front"/>
        </route>
    </router>
</config>

Create a block class which will be passed in .phtml file like-

<?php
/**
 * Copyright ©  All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace Gag\Front\Controller\Index;

class Index extends \Magento\Framework\App\Action\Action
{

    protected $resultPageFactory;

    /**
     * Constructor
     *
     * @param \Magento\Framework\App\Action\Context  $context
     * @param \Magento\Framework\View\Result\PageFactory $resultPageFactory
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory
    ) {
        $this->resultPageFactory = $resultPageFactory;
        parent::__construct($context);
    }

    /**
     * Execute view action
     *
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {
        return $this->resultPageFactory->create();
    }
}

In your .phtml file load the CMS page like-

<?php 

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$page = $objectManager->get('\Magento\Cms\Model\Page')->load('enable-cookies');
echo $objectManager->get('\Magento\Cms\Model\Template\FilterProvider')->getBlockFilter()->filter($page->getContent());

NOTE:- I have called object manager in .phtml file which is not right. You should use Factories or some better ways

Simple way is use PageHelper

<?php
declare(strict_types=1);

namespace Acme\StackExchange\Controller\Result;

use Magento\Cms\Helper\Page as PageHelper;
use Magento\Cms\Model\Page as PageModel;
use Magento\Framework\App\Action\HttpGetActionInterface;
use Magento\Framework\Controller\ResultInterface;

class Page implements HttpGetActionInterface
{
    /**
     * @var PageModel
     */
    protected $page;

    /**
     * @var PageHelper
     */
    protected $pageHelper;

    public function __construct(
        PageModel $page,
        PageHelper $pageHelper
    ) {
        $this->page       = $page;
        $this->pageHelper = $pageHelper;
    }

    /**
     * @return ResultInterface
     */
    public function execute()
    {
        return $this->pageHelper->prepareResultPage($this, 'enable-cookies');
    }
}

Add below code in your controller : 

$this->loadLayout();    
$block = $this->getLayout()->createBlock('block_type','name',array('template' => 'phtml_name'));
$this->getLayout()->getBlock('content')->append($block);
$this->renderLayout();