Here's some code where I "POST" to the gateway and the gateway responds with a value to indicate the transaction's status. 
No page is displayed on the gateway, it just returns a value.

Public Shared Function VerifiyPayment(ByVal TransactionID As String) As VerificationStatus
        Dim result As VerificationStatus = VerificationStatus.Unset

        Dim myRequest As HttpWebRequest = CType(HttpWebRequest.Create(GatewayParameters.VerificationURL), HttpWebRequest)
        myRequest.AllowAutoRedirect = False
        myRequest.Method = "POST"
        myRequest.ContentType = "application/x-www-form-urlencoded"

        Dim strToSend As String = String.Format("TransactionID={0}&ECOMRequestID={1}", TransactionID, GatewayParameters.ECOMRequestID)

        'Create post stream
        Dim RequestStream As Stream = myRequest.GetRequestStream()
        Dim SomeBytes() As Byte = Encoding.UTF8.GetBytes(strToSend)
        RequestStream.Write(SomeBytes, 0, SomeBytes.Length)
        RequestStream.Close()

        'Send request and get response
        Dim myResponse As HttpWebResponse = CType(myRequest.GetResponse(), HttpWebResponse)
        If myResponse.StatusCode = HttpStatusCode.OK Then
            'Obtain a 'Stream' object associated with the response object.
            Dim ReceiveStream As Stream = myResponse.GetResponseStream()
            Dim encode As Encoding = System.Text.Encoding.GetEncoding("utf-8")

            'Pipe the stream to a higher level stream reader with the required encoding format.
            Dim readStream As StreamReader = New StreamReader(ReceiveStream, encode)

            'Read result
            Dim Result As String = readStream.ReadLine()

            Select Case Result
                Case "0"
                    result = VerificationStatus.TransactionApproved
                Case "1"
                    result = VerificationStatus.TransactionDeclined
                Case "2"
                    result = VerificationStatus.TransactionNotFound
                Case "3"
                    result = VerificationStatus.SystemError
                Case Else
                    result = VerificationStatus.SystemError
            End Select

            'Close the response to free resources.
            myResponse.Close()   
        End If

End Function




After my customer enter's their payment info on the gateway's PCI Compliant page, it returns back to my site
with a "POST" (I think) and I have to read the parameters and process....

Private Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init
    Me.Page.EnableViewStateMac = False
End Sub

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Try
        If Page.IsPostBack = False Then

            TransactionID = GetRequestParam("TransactionID")
            ResponseCode = GetRequestParam("ResponseCode")
            ResponseText = GetRequestParam("ResponseText")
            ApprovalCode = GetRequestParam("ApprovalCode")
            AVSResultCode = GetRequestParam("AVSResultCode")
            PurchaseIdentifier1 = GetRequestParam("PurchaseIdentifier1")
            PurchaseIdentifier2 = GetRequestParam("PurchaseIdentifier2")

            hfTransactionID.Value = TransactionID
           
            .... Save the transaction ...

            'display the transaction info...
            With paymentInfo
                lblAccountNumber.Text = .AccountID
                lblCustomerName.Text = .AccountName
                lblTransactionID.Text = .Id
                lblTransactionDate.Text = .TransactionDateTime
                lblTransactionAmount.Text = sy.DisplayMoney(.TransactionAmount) & "   * Includes Convenience Fee"
                lblStatus.Text = .TransactionStatus
                lblApprovalCode.Text = .ApprovalCode
                lblMessage.Text = .ResponseText
            End With

        End If
    Catch exc As Exception        'Module failed to load
        ProcessModuleLoadException(Me, exc)
    End Try
End Sub

Private Function GetRequestParam(ByVal parameter As String) As String
    Dim result As String = ""
    Try
        result = Request.Params(parameter)
    Catch ex As Exception
    End Try
    Return result
End Function