I am trying to merge a static background with a GIF created using GAP in GIMP

[Ofnuts]

With interleave-layers, both images must have the same width and height, so you likely want to use Image>Canvas size on the animation. Once you have positioned the animation in the canvas, you can use ofn-layers-to-image-size (same place as interleave-layers) to make the layers fill the whole canvas.

Then to produce the image above, two possibilities:

1) the fixed part has a transparent hole, you insert it in "title" mode in the animation image (fixed image copied over each frame of the animation)

2) the animation is restricted to a circle and all the layers are transparent outside of the circle, you insert the fixed image in "sprite" mode in the animation.