GIF is not designed for high-quality images. Smooth vectors from Flash will end up pixelated in GIFs notno matter what you do. Photos and the like will generally look bad because of the reduced palette. Dithering can help, but the end result will still be much lower quality.
If you want an animation, you should really be using straight Flash and not GIFs (apart from very simple ones where the quality is not degraded). Or possibly a video format depending on your needs — most large gif-hosting sites seem to have moved to MP4 or WEBM for their primary data.