I can't use the gradient tool at all. I have a certain area selected, and try to use the shape gradient in that area, but I just can't! There's a circleslash on the icon. I tried every shape, selecting and deselecting, on every layer, and it just isn't working.
From your description you are trying to make a gradient on a color-indexed image (a GIF, possibly). The gradient tool cannot work on such images, you have to convert them to RGB (
Note however that the gradient generates many colors. When exporting again in a color-indexed format such as Gimp, all the colors in the image will be shoe-horned into the 256 colors of the color map. Many colors of your image will shift a bit, resulting in a grainy/pixellated look.