Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 113

2.5.5 Rotating an Image - Using the Rotate Tool
Figure 2.42
The angle was measured with the Measure tool, and now the measurement
needs to be entered in the Rotate dialog's Angle text box You can also use
the slider beneath the text box. Note that the desired rotation is counter-
clockwise; therefore, a minus sign is used before the numerical entry.