How to Make Payment Gateway in Magento 2

<?php $_order = new Mage_Sales_Model_Order();
$orderId = Mage::getSingleton('checkout/session')->getLastRealOrderId();
$_order->loadByIncrementId($orderId);

$amountToPay = $_order->getBaseGrandTotal();
?>

<form action='https://www.gateway.com/.../' method='post'>
<input id='AmountToPay' name='AmountToPay' value="<?php echo $amountToPay; ?>" type='hidden' />
<input id='Details2' name='Details2' value='<?php echo $orderId; ?>' type='hidden' />
</form>

and so on with the rest of the form fields...

The common tutorial for adding custom payment integration is still in process, but you can use sample-module-payment-gateway as an example.

Also, the off dev doc describes how to add a custom payment to the checkout page.

And a new implementation of the Braintree payment method is a good example of best practices for payment integrations.

UPD: Magento team has published payments methods documentation.