Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 237

3.9.3 Transforming a Selection
You've already used the Ellipse Select tool to create a shape on the new layer
titled egg . It will appear as a real circle only if you hold down the Shift key
while dragging the elliptic selection.
Use the Bucket Fill tool or choose Edit > Fill with FG Color to fill the object
with the red shade selected in the Color Editor (pure red is RGB 255.0.0).
First, drag a horizontal guide to the center of the circle. Then drag a
rectangular selection across the upper half of the red circle (lines within the
grid area) on the egg layer. This selection can be larger than the intended
object. The program will find the object borders automatically because the
remaining area is on a transparent layer.
Select the Scale tool . In the tool options, select the View > Image + Grid
menu item. Click on the image to access the Scale dialog. The transformation
grid that was initially limited to the selection surface now must be pulled
upward from the upper edge (see figure 3.58). Click the Scale button in the
Scale dialog to accept your changes.
Figure 3.58
A selected area on a layer can be transformed.