Magento – How to redirect from magento 2 checkout page to the 3rd party payment gateway api

You don't want to use any curl or server side stuff. This would mean the user submits his credentials to you and not the payment provider. Man in the middle for free.

Have a look on the paypal redirect: \Mage_Paypal_Block_Standard_Redirect.

Use a <form> with all the informations and submit it with JavaScript

There are lots of changes, need to your Payment method class.

1. Need to redirect to a Magento Custom Page.
2. Then From this Custom page submit a form which will hit payment gateways.
3. After Payment done/failure .paygate redirect to you a page

For 1st step1, you need do below change

add

   public function getCheckoutRedirectUrl()
    {
        return $this->_urlBuilder->getUrl('[RouteName]/[Controller]/[Action]');
    }

Your model class should be like this:

<?php

namespace Xyz\Abc\Model;
/* add by Amit Bera*/
use Magento\Sales\Api\Data\OrderPaymentInterface;
use Magento\Sales\Model\Order\Payment;
use Magento\Sales\Model\Order\Payment\Transaction;
use Magento\Quote\Model\Quote;
use Magento\Store\Model\ScopeInterface;
use Magento\Framework\Pricing\PriceCurrencyInterface;
/**
 * Bank Transfer payment method model
 *
 * @method \Magento\Quote\Api\Data\PaymentMethodExtensionInterface getExtensionAttributes()
 */
class XyzPaymentMethod extends \Magento\Payment\Model\Method\AbstractMethod
{
    const PAYMENT_METHOD_XYZ_CODE = 'xyzpaymentmethod';

    /**
     * Payment method code
     *
     * @var string
     */
    protected $_code = self::PAYMENT_METHOD_XYZ_CODE;

 /**
     * Availability option
     *
     * @var bool
     */
    protected $_isGateway = false;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canOrder = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canAuthorize = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canCapture = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canCapturePartial = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canRefund = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canRefundInvoicePartial = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canVoid = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canUseInternal = false;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canUseCheckout = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canFetchTransactionInfo = true;

    /**
     * Availability option
     *
     * @var bool
     */
    protected $_canReviewPayment = true;

    /**
     * Website Payments Pro instance
     *
     * @var \Magento\Paypal\Model\Pro
     */
    /**
     * Get instructions text from config
     *
     * @return string
     */
   /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;

    /**
     * @var \Magento\Framework\UrlInterface
     */
    protected $_urlBuilder;

    /**
     * @var \Magento\Checkout\Model\Session
     */
    protected $_checkoutSession;

    /**
     * @var \Magento\Framework\Exception\LocalizedExceptionFactory
     */
    protected $_exception;
     /**
     * @var PriceCurrencyInterface
     */
    protected $priceCurrency;   

   /**
     * @param \Magento\Framework\Model\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param \Magento\Framework\Api\ExtensionAttributesFactory $extensionFactory
     * @param \Magento\Framework\Api\AttributeValueFactory $customAttributeFactory
     * @param \Magento\Payment\Helper\Data $paymentData
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
     * @param Logger $logger
     * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
     * @param \Magento\Framework\Data\Collection\AbstractDb $resourceCollection
     * @param array $data
     * @SuppressWarnings(PHPMD.ExcessiveParameterList)
     */
      public function __construct(
        \Magento\Framework\Model\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\Api\ExtensionAttributesFactory $extensionFactory,
        \Magento\Framework\Api\AttributeValueFactory $customAttributeFactory,
        \Magento\Payment\Helper\Data $paymentData,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Payment\Model\Method\Logger $logger,
        /* add by Amit Bera */
        \Magento\Framework\UrlInterface $urlBuilder,
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Framework\Exception\LocalizedExceptionFactory $exception,
         \Magento\Store\Model\StoreManagerInterface $storeManager,
          PriceCurrencyInterface $priceCurrency,
        /* add by Amit Bera */
        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
        array $data = []
    ) {
        parent::__construct(
            $context,
            $registry,
            $extensionFactory,
            $customAttributeFactory,
            $paymentData,
            $scopeConfig,
            $logger,
            $resource,
            $resourceCollection,
            $data
            );



        $this->_storeManager = $storeManager;
        $this->_urlBuilder = $urlBuilder;
        $this->_checkoutSession = $checkoutSession;
        $this->_exception = $exception;         
      }

    public function getInstructions()
    {
        return trim($this->getConfigData('instructions'));
    }
     public function getCheckoutRedirectUrl()
    {
        return $this->_urlBuilder->getUrl('[RouteName]/[Controller]/[Action]');
    }


}

Step2

For create url you need a custom url which is need contoller file

For this follow inchoo block

http://inchoo.net/magento-2/how-to-create-a-basic-module-in-magento-2/

On this file at execute().. you can get curren placed order by below code

$LastorderObject=$this->_objectManager->get('Magento\Checkout\Model\Session');
        if (!$LastorderObject-> getLastRealOrder()->getId()) {
            return $this->resultRedirectFactory->create()->setPath('checkout/cart');
        }

then render block class with phtm file .. on phmtl submit form in hidden process nad redirect to payment gateway