How to create a custom WooCommerce Payment Gateway

gateway.php

<?php

// ...

function woo_payment_gateway() {
	class Woo_PayPal_Gateway extends WC_Payment_Gateway {
	  
	}
}

/**
 * Add Gateway class to all payment gateway methods
 */
function woo_add_gateway_class( $methods ) {
	
	$methods[] = 'Woo_PayPal_Gateway'; 
	return $methods;
}

add_filter( 'woocommerce_payment_gateways', 'woo_add_gateway_class' );

gateway2.php

<?php

// ...

function woo_payment_gateway() {
	class Woo_PayPal_Gateway extends WC_Payment_Gateway {

		/**
		 * API Context used for PayPal Authorization
		 * @var null
		 */
		public $apiContext = null;
		
		/**
		 * Constructor for your shipping class
		 *
		 * @access public
		 * @return void
		 */
		public function __construct() {
		    	$this->id                 	= 'woo_paypal'; 
		    	$this->method_title       	= __( 'Woo PayPal', 'woodev_payment' );  
		    	$this->method_description 	= __( 'WooCommerce Payment Gateway for ibenic.com tutorial', 'woo_paypal' );
		    	$this->title              	= __( 'Woo PayPal', 'woo_paypal' );

			$this->has_fields = false;

			$this->supports = array( 
				'products'
			);

			$this->get_paypal_sdk();

		   	// Load the settings.
        		$this->init_form_fields();
        		$this->init_settings();
		    	$this->enabled 		= $this->get_option('enabled');
		    
		    	add_action( 'check_woopaypal', array( $this, 'check_response') );

		    	// Save settings
  			if ( is_admin() ) {
  				// Versions over 2.0
  				// Save our administration options. Since we are not going to be doing anything special
  				// we have not defined 'process_admin_options' in this class so the method in the parent
  				// class will be used instead
  				add_action( 'woocommerce_update_options_payment_gateways_' . $this->id, array( $this, 'process_admin_options' ) );
  			}	
		}
	}
}

// ...

gateway3.php

<?php

// ...

private function get_paypal_sdk() {
	require_once WOO_PAYMENT_DIR . 'includes/paypal-sdk/autoload.php';
}

gateway4.php

<?php

// ...

public function init_form_fields() {
	$this->form_fields = array(
		'enabled' => array(
			'title' => __( 'Enable', 'woo_paypal' ),
			'type' => 'checkbox',
			'label' => __( 'Enable WooPayPal', 'woo_paypal' ),
			'default' => 'yes'
		),
		'client_id' => array(
			'title' => __( 'Client ID', 'woo_paypal' ),
			'type' => 'text',
			'default' => ''
		),
		'client_secret' => array(
			'title' => __( 'Client Secret', 'woo_paypal' ),
			'type' => 'password',
			'default' => ''
		),
	);
}

gateway5.php

<?php

// ...

private function get_api_context(){

	$client_id =  $this->get_option('client_id');
	$client_secret =  $this->get_option('client_secret');
	$this->apiContext = new ApiContext(new OAuthTokenCredential(
		$client_id,
		$client_secret
	));
}

gateway6.php

<?php

// ...

public function process_payment( $order_id ) {

	global $woocommerce;
	$order = new WC_Order( $order_id );

	$this->get_api_context();

	$payer = new Payer();
	$payer->setPaymentMethod("paypal");
	
	$all_items = array();
	$subtotal = 0;
	// Products
	foreach ( $order->get_items( array( 'line_item', 'fee' ) ) as $item ) {

		$itemObject = new Item();
		$itemObject->setCurrency( get_woocommerce_currency() );

		if ( 'fee' === $item['type'] ) {

			$itemObject->setName( __( 'Fee', 'woo_paypal' ) );
			$itemObject->setQuantity(1);
			$itemObject->setPrice( $item['line_total'] ); 
			$subtotal += $item['line_total'];
		} else {

			$product          = $order->get_product_from_item( $item );
			$sku              = $product ? $product->get_sku() : '';
			$itemObject->setName( $item['name'] );
			$itemObject->setQuantity( $item['qty'] );
			$itemObject->setPrice( $order->get_item_subtotal( $item, false ) );
			$subtotal += $order->get_item_subtotal( $item, false ) * $item['qty'];
			if( $sku ) {
				$itemObject->setSku( $sku );
			}  
		}

		$all_items[] = $itemObject;
	}


	 
	$itemList = new ItemList();
	$itemList->setItems( $all_items );
	// ### Additional payment details
	// Use this optional field to set additional
	// payment information such as tax, shipping
	// charges etc.
	$details = new Details();
	$details->setShipping( $order->get_total_shipping() )
	    ->setTax( $order->get_total_tax() )
	    ->setSubtotal( $subtotal );

	$amount = new Amount();
	$amount->setCurrency( get_woocommerce_currency() )
	    ->setTotal( $order->get_total() )
	    ->setDetails($details);

	$transaction = new Transaction();
	$transaction->setAmount($amount)
	    ->setItemList($itemList)
	    ->setInvoiceNumber(uniqid());

	$baseUrl = $this->get_return_url( $order );

	if( strpos( $baseUrl, '?') !== false ) {
		$baseUrl .= '&';
	} else {
		$baseUrl .= '?';
	}

	$redirectUrls = new RedirectUrls();
	$redirectUrls->setReturnUrl( $baseUrl . 'woopaypal=true&order_id=' . $order_id )
	    ->setCancelUrl( $baseUrl . 'woopaypal=cancel&order_id=' . $order_id );

		$payment = new Payment();
	$payment->setIntent("sale")
	    ->setPayer($payer)
	    ->setRedirectUrls($redirectUrls)
	    ->setTransactions(array($transaction));

	try {

	    $payment->create($this->apiContext);

	    $approvalUrl = $payment->getApprovalLink();

	    return array(
			'result' => 'success',
			'redirect' => $approvalUrl
		);

	} catch (Exception $ex) {

		wc_add_notice(  $ex->getMessage(), 'error' );
	}

	return array(
			'result' => 'failure',
			'redirect' => ''
	);

}

paypal.php

<?php

// ...

use PayPal\Rest\ApiContext;
use PayPal\Auth\OAuthTokenCredential;
use PayPal\Api\Amount;
use PayPal\Api\Details;
use PayPal\Api\ExecutePayment;
use PayPal\Api\Item;
use PayPal\Api\ItemList;
use PayPal\Api\Payer;
use PayPal\Api\Payment;
use PayPal\Api\PaymentExecution;
use PayPal\Api\RedirectUrls;
use PayPal\Api\Transaction;

plugin.php

<?php

/**
 * Plugin Name: Custom WooCommerce PayPal Gateway
 */

plugin2.php

<?php

// ...

define( 'WOO_PAYMENT_DIR', plugin_dir_path( __FILE__ )); 

add_action( 'plugins_loaded', 'woo_payment_gateway' );

response.php

<?php

// ...

add_action( 'init', 'check_for_woopaypal' );

function check_for_woopaypal() {
	if( isset($_GET['woopaypal'])) {
	  // Start the gateways
		WC()->payment_gateways();

		do_action( 'check_woopaypal' );
	}
	
}

response2.php

<?php

// ...

public function check_response() {
	global $woocommerce;
	 
	if( isset( $_GET['woopaypal'] ) ) {
     
		$woopaypal = $_GET['woopaypal'];
		$order_id = $_GET['order_id'];

		if( $order_id == 0 || $order_id == '' ) {
			return;
		}

		$order = new WC_Order( $order_id );

		if( $order->has_status('completed') || $order->has_status('processing')) {
			return;
		}

		if( $woopaypal == 'true' ) {
			$this->get_api_context();

			$paymentId = $_GET['paymentId'];
			$payment = Payment::get($paymentId, $this->apiContext);

			$execution = new PaymentExecution();
			$execution->setPayerId($_GET['PayerID']);

			$transaction = new Transaction();
	    		$amount = new Amount();
	    		$details = new Details();

	    		$subtotal = 0;
			// Products
			foreach ( $order->get_items( array( 'line_item', 'fee' ) ) as $item ) {

				if ( 'fee' === $item['type'] ) {

					$subtotal += $item['line_total'];
				} else {

					$subtotal += $order->get_item_subtotal( $item, false ) * $item['qty'];
			
				}
			}

		  	$details->setShipping( $order->get_total_shipping() )
			 ->setTax( $order->get_total_tax() )
			 ->setSubtotal( $subtotal );
		 
			
  			$amount = new Amount();
  			$amount->setCurrency( get_woocommerce_currency() )
  				 ->setTotal( $order->get_total() )
  		   		->setDetails($details);
  
      			$transaction->setAmount($amount);
  
      			$execution->addTransaction($transaction);

			try { 
			
			      $result = $payment->execute($execution, $this->apiContext);
			      
			} catch (Exception $ex) { 
			
			      $data = json_decode( $ex->getData());
			      
			      wc_add_notice(  $ex->getMessage(), 'error' );
			
			      $order->update_status('failed', sprintf( __( '%s payment failed! Transaction ID: %d', 'woocommerce' ), $this->title, $paymentId ) . ' ' . $ex->getMessage() );
			      return;
			}

  	  		// Payment complete
	  		$order->payment_complete( $paymentId );
	  		// Add order note
	  		$order->add_order_note( sprintf( __( '%s payment approved! Trnsaction ID: %s', 'woocommerce' ), $this->title, $paymentId ) );
	  
	  		// Remove cart
	  		$woocommerce->cart->empty_cart();
  
  			}

		  	if( $woopaypal == 'cancel' ) { 
		  		$order = new WC_Order( $order_id );
		  		$order->update_status('cancelled', sprintf( __( '%s payment cancelled! Transaction ID: %d', 'woocommerce' ), $this->title, $paymentId ) );
		  	}
  	}
  	return;
}

@igorbenic do you happen to know why the gateway class is nested in a function?

function woo_payment_gateway() {
    class Woo_PayPal_Gateway extends WC_Payment_Gateway {}
}

@igorbenic do you happen to know why the gateway class is nested in a function?

function woo_payment_gateway() {
    class Woo_PayPal_Gateway extends WC_Payment_Gateway {}
}

because of this
add_action( 'plugins_loaded', 'woo_payment_gateway' );

Thanks @TheFedGuy