cannot pass 'this' as a ref or out parameter because it is read-only

My class passes itself to a method in another class. This external method (last line of code below) changes the object I'm passing accordingly, returns control, and moves on in a happy fashion. But the last line ThisPaymentGateway.Process(ref this);says it at design time:

cannot pass 'this' as a ref or out parameter because it is read only***

There is no read-only property there. How can I fix this?

using System;

namespace Something
{
  namespace Finance
  {
    namespace Donations
    {
      public class Electronic1 : Donation
      {
        private PaymentGateway ThisPaymentGateway { get; set; } = new PaymentGateway();

        public void RunController()
        {
          if (DonorType.ToUpper() == "IND" && (PaymentMethod.ToUpper() == "CREDIT CARD" || PaymentMethod.ToUpper() == "ECHECK"))
          {
            ThisPaymentGateway.Process(ref this);
          }
        }
      }
    }
  }
}