Where a gable end is simply the end wall of a saddle roof, the wall is usually cavity brickwork or blockwork from the base to the roof.
Where a gable end is formed above a portico, doorway or window screen it may be formed by other materials that will eventually be rendered or finished in matchboard, shiplap etc.
If you wish to form such a gable in blockwork, bearing in mind the weight to be supported above windows which are not intended to bear any weight, you will need substantial timber or steel girders. Your architect will provide guidance and you will probably need a structural engineer to provide the design and calculations.
These pages show in photograps how such a gable is prepared. It is fairly time consuming in labour and requires materials that are significant in cost.
Click on the photos for a larger view