This snippet demonstrates how to interact with each layer in GIMP

Script_Fu_for-each-layer.scm

(define (for-each-layer image drawable)
  (let* (
          (layers (gimp-image-get-layers image)) ; The list of all layers of image
          (all-layers (cadr layers))             ; The list of all layers
          (count-layers (- (car layers) 1))      ; The number of layers contained in the image -1 (scheme is zero indexed)
        )

        (gimp-image-undo-group-start image)
          (while (>= count-layers 0)
            (let ((current-layer (aref all-layers count-layers))) ; Catches current layer
              ; Do your work with "current-layer"
            (set! count-layers (- count-layers 1)))
          )
        (gimp-image-undo-group-end image)
        (gimp-displays-flush)
  )
)