ffmpeg -i in.mov -s 600x400 -pix_fmt rgb24 -r 10 -f gif - | gifsicle --optimize=3 --delay=3 > out.gif
Notes on the arguments:
-r 10
tells ffmpeg to reduce the frame rate from 25 fps to 10-s 600x400
tells ffmpeg the max-width and max-height--delay=3
tells gifsicle to delay 30ms between each gif--optimize=3
requests that gifsicle use the slowest/most file-size optimization