Is this what you want? To call a geoprocessing tool and see its interface? This code is in VBA.
Public Sub OpenCalculateTool()
' Hook into Toolbox and get the tool
Dim pUID As New UID
pUID = "esriGeoprocessingUI.ArcToolboxExtension"
Dim pArcToolboxExtension As IArcToolboxExtension
Set pArcToolboxExtension = Application.FindExtensionByCLSID(pUID)
Dim pArcToolbox As IArcToolbox
Set pArcToolbox = pArcToolboxExtension.ArcToolbox
Dim pGPTool As IGPTool
Set pGPTool = pArcToolbox.GetToolbyNameString("CalculateField")
' Create messages, required by Invoke method
Dim msgs As IGPMessages
Set msgs = New GPMessages
' Get existing parameter structure
Dim pArray As IArray
Set pArray = pGPTool.ParameterInfo
' Declare Parameter interfaces
Dim pGPParameter As IGPParameter
Dim pGPDataType As IGPDataType
Dim pGPParameterEdit As IGPParameterEdit
' Set the parameters of the tool
Set pGPParameter = pArray.Element(0)
Set pGPParameterEdit = pGPParameter
Set pGPDataType = pGPParameter.DataType
Set pGPParameterEdit.Value = pGPDataType.CreateValue("a") ' Featurelayer name
Set pGPParameter = pArray.Element(1)
Set pGPParameterEdit = pGPParameter
Set pGPDataType = pGPParameter.DataType
Set pGPParameterEdit.Value = pGPDataType.CreateValue("xx") ' Field
Set pGPParameter = pArray.Element(3)
Set pGPParameterEdit = pGPParameter
Set pGPDataType = pGPParameter.DataType
Set pGPParameterEdit.Value = pGPDataType.CreateValue("VB")
' Open the tool
Dim pGPToolCommandHelper As IGPToolCommandHelper2
Set pGPToolCommandHelper = New GPToolCommandHelper
pGPToolCommandHelper.SetTool pGPTool
pGPToolCommandHelper.InvokeModal 0, pArray, True, msgs
End Sub