GiD reference manual : Vtk 

Menu : Files->Export→Vtk...


With this option it is possible to write Vtk files, in xml format (with .vtu or .vtp extension), with ASCII, binary and compressed options

This exporter is implemented as a Tcl plugin, based on the Vtk package library.

Postprocess meshes and results are exported, of the current analysis and step, or all writing several files.