The gateway is defined on a per network card level, so this code will get you the Gateway for each of the NICs installed on a machine:

Code:
        Dim sb As New StringBuilder
        sb.AppendLine()
        For Each n As NetworkInterface In NetworkInterface.GetAllNetworkInterfaces()
            sb.AppendLine(n.Description)
            sb.AppendLine("MAC:" & n.GetPhysicalAddress().ToString())
            For Each u As UnicastIPAddressInformation In n.GetIPProperties().UnicastAddresses
                sb.AppendLine("IP:" & u.Address.ToString())
            Next
            For Each g As GatewayIPAddressInformation In n.GetIPProperties().GatewayAddresses
                sb.AppendLine("Gateway:" & g.Address.ToString())
            Next

        Next


        MessageBox.Show(sb.ToString())