Hi
You could save your animation as a bitmap sequence, then use another software to make your GIF animation from the bitmaps... It could take time to do so, but you would have a good control on the result...
And that's the only solution i think of for your problem.
Anyway, i hope for you someone else will find an easier/faster solution than my suggestion.
Good luck and have fun with Bryce! =)
Jean-Luc