This is a modified version of Thomas Hourdel's Moments GIF Recorder for Unity.
Moments is a quick GIF replay recorder for Unity3D. It automatically records the last few seconds of gameplay and lets you save to a GIF file on demand, like the game TowerFall Ascension does.
MomentsPreview adds code to allow you to copy the queue of RenderTextures that Moments uses to buffer and generate its GIFS, as well as all necessary playback information. You can then use this set of RenderTextures to display a preview of the GIF by cycling through the RenderTextures on a RawImage UI element. The demo in MomentsPreview currently does not account for playback speed, but the CapturedReplay object contains all the necessary data and accessor methods to adjust playback speed.
Tested with Unity 5.4.
Drop the Moments Recorder folder in your project and add the Recorder script to your camera (or select your camera and use Component -> Miscellaneous -> Moments Recorder).
The included demo should get you started. For more advanced features, browse the Moments.Recorder source code, it's heavily commented.
Here's a preview of the output quality.
Zlib (see License.txt)