Introduce New Payment Gateway:

add_filter("payment_method", "\WPDMPP\Libs\PaymentMethods\wpdmpp_new_payment_method"); 
function wpdmpp_new_payment_method($methods) { 
    $methods[] = "PaymentMethodClassName"; return $methods; 
}

Complete Implementation:

namespace WPDMPP\Libs\PaymentMethods;

add_filter("payment_method", "\WPDMPP\Libs\PaymentMethods\wpdmpp_new_payment_method"); 

function wpdmpp_new_payment_method($methods) { 
    $methods[] = "PaymentMethodClassName"; 
    return $methods; 
}

if (!class_exists('PaymentMethodClassName') && class_exists('\WPDMPP\Libs\CommonVars')) {
    class PaymentMethodClassName extends \WPDMPP\Libs\CommonVars
    {
        var $TestMode;
        var $GatewayUrl = "https://gatewat.url";
        var $GatewayUrl_TestMode = "https://sandbox.gateway.url";        
        var $ReturnUrl;
        var $NotifyUrl;
        var $CancelUrl;         
        var $Enabled;       
        var $VarName;
        var $GatewayName = "Gateway Name";

        function __construct()
        {
                $this->TestMode = $TestMode;            
                $this->Enabled = get_wpdmpp_option('PaymentMethodClassName/enabled');
                $this->ReturnUrl = wpdmpp_orders_page();                
                $this->NotifyUrl = home_url('?action=wpdmpp-payment-notification&class=PaymentMethodClassName');
                $this->CancelUrl = wpdmpp_cart_page();
                $this->ParameterName = get_wpdmpp_option('PaymentMethodClassName/VariableName');
                $this->TestMode = get_wpdmpp_option('PaymentMethodClassName/Mode');
                if($this->TestMode=='1') $this->GatewayUrl = $this->GatewayUrl_TestMode;
                $this->Secret_word = get_wpdmpp_option('PaymentMethodClassName/secret_word');
                $this->Currency = wpdmpp_currency_code();
            }
          
        }


        function configOptions()
        {

            $options = array(

                'Mode'=> array(
                    'label'         =>      __("Payment Gateway Mode:","text-domain"),
                    'type'          =>      'select',
                    'options'       =>      array('1' => 'Live', '0' => 'Test'),
                    'selected'      =>      $this->TestMode
                ),
                'VarName'=> array(
                    'label'         =>      __("Text Label","text-domain"),
                    'type'          =>      'text',                    
                    'placeholder'   =>      '',
                    'value'         =>      $this->VarName
                )
                // Add more variables as your payment gateway settings requires
                
            );

            return $options;


        }

        function showPaymentForm()
        {
            global $current_user;             
            $sbtn = "";
            $hide = "";
            if ($AutoSubmit == 1) $hide = "display:none;'";
            else $sbtn = "<button type='submit'>Pay Now!</button>";
            $this->Amount = number_format($this->Amount, 2);             
            $Form = "
                    Payment Form / Code to redirect to payment gateway... 

        ";
            return $Form;


        }


        function verifyNotification()
        {

         //IPN Code to verify payment and retun data true/false
          $valid = true;
           return $valid;
        }
    }
}