How to Rotate Selections in GIMP
- 1). Use any of GIMP's selection tools -- any of the first six tools in the Toolbox -- to select a portion of your image.
- 2). Click the Rotate button, illustrated by two blue rectangles with arrows indicating the axis of rotation. The Rotate tool will be selected and rotation options will appear in the lower pane of the Toolbox.
- 3). Click the "Layer" button under "Transform" in the lower pane, if it's not already selected. This tells the tool to rotate a particular layer of the image -- in this case, the selection.
- 4). Move the cursor over your selection, and then left-click. The Rotate dialog box will appear. You can hold down the left button and move the mouse to rotate the selection manually, or you can use the controls in the Rotate box to select a precise angle of rotation.
- 5). Click "Rotate" to complete the rotation.
Source...