res = request.GET['paymentid'] will raise a KeyError if paymentid is not in the GET data.
Your sample php code checks to see if paymentid is in the POST data, and sets $payID to '' otherwise:
$payID = isset($_POST['paymentid']) ? $_POST['paymentid'] : ''
The equivalent in python is to use the get() method with a default argument:
payment_id = request.POST.get('payment_id', '')
while debugging, this is what I see in the
response.GET: <QueryDict: {}>,request.POST: <QueryDict: {}>
It looks as if the problem is not accessing the POST data, but that there is no POST data. How are you are debugging? Are you using your browser, or is it the payment gateway accessing your page? It would be helpful if you shared your view.
Once you are managing to submit some post data to your page, it shouldn't be too tricky to convert the sample php to python.