By definition 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 design 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.

A higher resolution GIF image or animation can be achieved by increasing the size of your ad up to 960px. You can do this when you start creating a design or later on with smart resize.