When exporting images, you need to be aware that most image formats have limitations. A limitation of gif images is, that it supports a maximum of 256 colors. For animated gif, this is 256 colors per frame; however, GIMP does not support exporting each frame with a different set of 256 colors. When exporting to gif, GIMP reduces the number of colors in your image to 256 by combining more or less similar colors together. This can cause noticeable changes in your image.

To fix this, you will have to choose a different image format that does support more colors; or convert your image to Indexed Mode and manually adjust any colors before exporting to gif.