A thing to convert a Gimp image of multiple layered sprites into a sprite sheet.
The way I make pixel art sprites is this: imagine you're drawing, say, a knight. I have all the frames for that knight in one great big Gimp XCF image, organised into groups.
So there's a layer group called "Running"; this defines a whole sprite, in multiple frames. In there, there's a set of layer groups, one per frame, called "Running 1" to "Running 9" (for example). In "Running 1", there's one or more layers which all go together to form that one single frame. This works out neatly because you can make an individual frame out of a few different parts, and edit those parts separately. This makes it easy to copy unchanging parts from one frame to the next, while just changing the bits that are different.