Hi Omar, welcome to the forum.
Unfortunately, it isn't that simple to create a stretchable GDL object. You would have to know a little about GDL editing to add the correct parameters in the right places into the code.
Could you resave the object into 3 separate parts? Or maybe make the sides using a complex profiled column, which you could stretch?
Perhaps post up a picture and we could suggest a different/better way of achieving what you are looking for.