interleave-layers script requires that the stack of layers and the inserted layers all have the same size. So you need to:
Image>Canvas size to enlarge the animation canvas to the final size, and select
All layers. You can position your layers in the bigger canvas at that point (
Center button, or drag them in the preview widget).
- Add a layer to the image (it will normally assume the size of the canvas), filled with the background color
- Make that layer invisible
Image>Interleave layers>Interleave layers (sprite mode) with:
Layer: the background layer
This creates a new image.
Note that if your layers have no alpha channel, and the background color in the toolbox swashes is already set to your grey-like color, then just the first step of all this (
Image>Canvas size) will be necessary, since Gimp extends alpha-less layers with the background color.