Giza by default suppresses the display of 3D shapes while in the 2D Plan view.
If you would like to see the 3D shapes while in the plan view you can disable this by going to the Tools menu and choosing options. Then select the Display tab. Remove the check mark next to “Suppress Display of all 3D Data in 2D Windows” and press Apply and Ok. You should then see 3D data also while in the 2D display.
The other option would be to make sure the DWG file you are importing has both a 2D Graphic and the 3D Face model stacked on top each other. That way with the default setting Giza would show only the 2D portion of the file in plan view, and then show only the 3D portion while in 3D View.