Combine multiple GIFs together
Make a GIF, where two or more GIFs play simultaneously. One can be on top of another.
In short, open the main background image (or create an empty one with File - New). Then, open two or more GIFs (using File - Open) and duplicate their layers into the main image. You can move them, resize, rotate etc. If you export it now, animations will paly one after another.
Now, simply press Layer - Animation - Merge. The structure will change as illustrated below.
Now, just press File - Export as - GIF, Save, and you are done :)