Gid have a function to create automatic 6-sided volumes (Geometry Create Volume Automatic Struct).

In the current version, several volumes can be created simultaneously, selecting all the boundary surfaces. GiD automatically detects the holes.

To add manually a hole to a volume, invoke the menu: Geometry->Create->Volume->Hole, then select the outer volume and then select the volumes of the hole.

Option Search in volume creation permits to select one surface and GiD tries to find one volume that contains this surface.