The GIF format supports up to 8 bits per pixel, thus allowing a single image to reference a palette of up to 256 distinct colors.
If your banner has more than 256 (or 128) colors, the most used colors in your image will show correctly, and all the other colors will "dither" which is a dotting process, trying to match a middle color by using half pixels of one close color and half pixels of another closer color.
Also, the built in effect won’t show in GIF banners. The banner will play its slides as regular images. So, the final .gif file will include a screenshot for each slide. Read more about it here.