Different image sizes

The images you provided have different sizes. In the merging process the images get resized by diffrent amounts. This might lead to information and quality loss.

Too much colors used

The images contain more than 255 colors. The gif format is limited to 256 colors and one color is reserved for internal processes. The colors get reduced by merging similar colors. You can select the merging algorithm by changing the dither option.

Semi transparent colors

The images contain pixels with semi transparent colors. This tool does not support semi transparent colors. The colors are converted to transparent or opaque. You can change the behavior by selecting the Alpha Channel Handling option. This warning might also occur due to scaling of images or the image smoothing option.

No transparent pixels

You've selected the transparent color to be the transparency key. This requires atleast one transparent pixel per frame. Otherwise all colors get keyed out.