1 Create a new 200x200 pixels, black background image ( File > New ).
Add a new layer ( Layers > New Raster Layer ).
Pick the selection tool and create a square selection as shown in the image.
2 Set your foreground color to red (#ff0000). Then, Floodfill current selection with red.
3 Pick the Freehand Selection tool (see selection settings below). Remove the lower selection as shown in the image below. You can do this by holding down the CTRL key, click your mouse on the all the four corners as shown in the image.
Selection Settings = Point to point Feather = 0 Anti-Alias = checked