You can do this by pressing the Eye icon by the Background in the layers tab. Once done, export the image as a PNG to retain the background transparency. You can now place the image on top of ...

In a similar manner, add multiple layers that you want in your output GIF. If you want to import some transparent images to your GIF animation, you can do so using the File > Open as Layers option.

In order to save the layered image in a single-layer graphics format such as TIFF or JPEG, the image is said to be "flattened." An Adobe PDF file is also flattened to remove a transparency layer ...