GiD installation include by default a vtk Tcl package (VTK version 8.0) at /scripts/vtk
(as you said is is a wrapper of the vtk package with Tcl bindings to be used with this scripting language)
but the VTK library is not a GUI library of widgets (like Tk or ttk)
This library is intended to manipulate scientific data, create rendered images, and do some operations (similar of the ones of GiD postprocess to calculate iso-surfaces, streamlines, and others)
In fact GiD is not using VTK internally, we have our own code.
For us it is an auxiliary Tcl package that we are using only to read/write VTK format files (mesh+results)
postprocess menu: Files->Import->Vtk and Files->Export->Vtk
The Vtk import/export features are implemented as an ‘external Tcl GiD plugin’ in
\plugins\Import\vtk\vtk.tcl
You can have a look to this file to see how to use in Tcl some commands of the vtk package.
For other Vkt commands you must find documentation at the VKT webpage