It's known that you can use vbscript in GTL but there's less information about using GTL within vbscript. This is an example on how to use the result from a GTL template within your vbscript.

Sub GTLExample()
    Dim PD As PdCommon.Application
    Dim Model As PdLDM.Model
    Dim S As String
    Dim TTI As TemplateTargetItem

    Set PD = CreateObject("PowerDesigner.Application")
    Set Model = GetModelFromName(PD, "My Model Name")
    Set TTI = Model.FindMetaExtensionByName(Model, PdCommon.cls_TemplateTargetItem, "Ordered Entities")
    S = Model.EvaluateTemplate("Ordered Entities", TTI)
    MsgBox S
End Sub

The code finds the "Ordered Entities" template in the extension and evaluates the GTL.