Menu: View->Clip planes...
This window lets you hide the front or back of the view.
Clip planes is a way to prevent GiD from drawing elements of the geometry or mesh that are either very close to or far from the observer.
By moving the Near plane (blue square in the sketch), the geometry that is closer to the viewer is clipped away, while moving the Far plane (red square in the sketch) culls the geometry that is further from the viewer.
Selecting Move planes together leaves a constant distance between both planes, and so moving one plane it moves de the second one at the same time.
Note: All this information is reset when performing a zoom frame. (see Zoom).