••• A newer release with an important bugfix is available •••
./gifsicle --lossy=80 -o lossy-compressed.gif input.gif
--lossy argument to suit quality (30 is very light compression, 200 is heavy).
It works best if the input image/anim is high quality and doesn't have heavy dithering (lossy compression will add its own dithering).
Big thanks to Nico Bergemann for finding the bug that was stopping this release.