Did you create a payment from your payment method and add that payment to the cart?

Yes, I did.  I am not sure why it doesn't seem to work in a non-Quicksilver project.

It's pretty damn complex from what I've seen worked for a payments processor so far.

There is an insane number of moveable parts that are required to perfectly mesh for even simple transactions. Not much because of what the code does. The code base on the API alone is massive and basically contains a tonne of interlocking and interdependent data transmission pieces.

The pre-existing payment management systems and norms you will need to work with are not exactly the easiest to use, as others have already said.

If you are implementing your payment gatway as a Generic payment gateway, you may solve this issue by handling "routing" yourself.

Create a class that inherit from IPaymentProcessor, override ProcessPayment and then route to the different gateways depending on payment name. 

public PaymentProcessingResult ProcessPayment(IOrderGroup orderGroup, IPayment payment, IShipment shipment)
        {
            if (payment.PaymentMethodName == PayExPaymentMethod.PaymentMethodSystemName)
            {
                return _payExPaymentGateway.ProcessPayment(orderGroup, payment);
            }

            if (payment.PaymentMethodName == VippsConstants.VippsSystemKeyword)
            {
                return _vippsPaymentGateway.ProcessPayment(orderGroup, payment);
            }

            return _genericPaymentGateway.ProcessPayment(orderGroup, payment);
        }

I am assuming you have created payment options in Commerce Manager using the payment plugin you want to use. During checkout or setting payment method to your cart, cart requires you to use IPaymentMethod to create your payment and add the payment to your cart. Without this, it does not trigger the payment gateway/ plugin you have created. 

Code would look something like below:

paymentMethod.CreatePayment(cart.GetTotal().Amount, cart);
cart.AddPayment(payment, _orderGroupFactory);

Whichever implementation you have for IPaymentMethod, ensure you have implemented your create payment to use your custom logic to assign correct payment method id amount etc. Usually a create payment method looks like this.

        public override IPayment CreatePayment(decimal amount, IOrderGroup orderGroup)
        {
            ICreditCardPayment payment = orderGroup.CreateCardPayment(OrderGroupFactory);
            payment.PaymentMethodId = PaymentMethodId;
            payment.PaymentMethodName = SystemKeyword;
            payment.Amount = amount;
            payment.Status = PaymentStatus.Pending.ToString();
            return payment;
        }

Hope this helps!

~ Sujit