Gif with multiple gifs playing simultaneously

[Ofnuts]

The first difficulty is that all merged GIFs should have the same number of frames. If this is not the case there is a solution: replicate each animation until it reaches the least common multiple of the set. For instance, if you have an animation with 6 frames and one with 4 frames, the LCM is 12, and you can make a 12-frames animation by combining 2 copies of the 6-frames one with 3 copies of  the 4-frames one. If you have many GIFs the LCD can be quite big.  

If you use interleave-layers:

interleave-layers requires all the merged animations to have the same image size, so you have to evaluate the total size of the canvas, and in each animation:

• Image>Canvas size to set the image size to the required final size
  
• Use the "Offset" widget or the Position widget to move the animation to the required position in the canvas
  
• Make sure you have Resize layers: all layers, and Fill with: Transparency
  

Once this is done, applying interleave-layers is easy, just keep all the defaults. Advice: try your hand with an image that merges two or three animation first.

Interleave-layers requires the images to b in RGB mode (GIF are typically loaded as color-indexed mages) so you will likely have to use Image>Mode>RGB.

Which naturally introduces a big warning:

• GIFs are color-indexed, the color of each pixel is an index into a "colormap" that contains at most 256 colors
  
• Unless all your GIFs use the same colormap, your combined animation will have much more than 256 colors
  
• When exporting to GIF, if will be color-indexed again and reduced to 256 colors. Quality loss with grainy/pixellated images will ensue
  

This isn't a problem with interleave-layers or Gimp, this is a restriction of the GIF format.