The easiest way to create a 1/2 circle surface in GiD is to draw the diameter line and the arc, and create a surface using these 2 as contour. If you already have a circle and its perimeter has the start+end point where you need, you can also delete the surface, divide the curve (circle), connect the old and new point through a line, create the surface, and either delete the 1/2 circle arc you have not used, or create the other 1/2 circular surface.