I tried rendering a single frame from 5 seconds in SIF file rainbows.sif. First I used GIF format, and it came out looking like this:
$ synfig --time 5 -o rainbows.gif rainbows.sif
At first I thought this was another gradient bug, but then I tried PNG format, and it looked a lot better:
$ synfig --time 5 -o rainbows.png rainbows.sif
In the #synfig IRC channel, pabs3 pointed out that .gif images have a limit of 256 colours in their palette, and the above .png image has 1883 unique colours in it, so that is more than likely why the .gif version looks so rough. However, even with the 256 colour limit I would expect you can do a lot better than the image above.