Export to PDF. Open in Photoshop. Save as GIF.
In case you don't got Photoshop (which is possible, Steve), you can also export to PNG and then convert to GIF using any free picture converter you got (Windows Paint or GIMP or whatever).
You can also export as .eps or jpeg then open in 'photoshop' or 'illustrator' then save for web in both as mention in earlier replies.
Steve's way is the better way of doing this.
If you don't have photoshop then there are other alternatives that will open a PDF and allow to save to GIF.
PDF to GIF on zamzar works well. http://www.zamzar.com/convert/pdf-to-gif/