So now each layer of the image should represent one frame. You can copy a frame by selecting the appropriate layer, doing a Select All and Copy.
The way I'd make a spritesheet is by making a new, really big blank image and one by one copy & paste each frame of the animation into it. When using GIMP, it would help to set the grid in your spritesheet to match the size of your animation; that will make it a lot easier to line up.
As long as the tiles are laid out in a grid and arranged in a reading order, the game should recognize it. So the spritesheet can even be all 1 row.