Here is a simple one-line command to turn a video into fast GIF:
ffmpeg -i input.mp4 -vf "setpts=0.5*PTS,scale=480:-1" -r 15 output.gif
How this works:
-i input.mp4: specifies your input video.
setpts=0.5*PTS: doubles the playback speed (half the presentation
If your need is simple, you can also just do
convert video.mp4 video.gif