Earlier today I had to rotate an animated GIF. I thought I was in for a tough ride but it turns out to be pretty easy to do.

All you need to do is to open the animated GIF in GIMP. Each frame of the animation is shown as a separate layer). Next you can rotate the image using "Image > Transform > Rotate" and save it. During the save GIMP asks whether you want to flatten the image or save it as an animation.

Straightforward.