Need Help! No smoothed animation.

here there, I wanna make an animated gif of a rotating planet. That rotates 360 ° and then again, so I want it to be looped, but if I add a key at frame 1 with 0° and at frame 160 with 360°. If I playback the animation It will end and start slowly. At the end the animation look worse, I want it to start inmediatly so there will be a nice unending loop in the final gif.
Is this possible and how?

Select the IPO in the Curve Editor, press E and select Extrapolation. Done, and done.

Thank you very much!