How can I do a limited/masked gradient fill in GIMP?

[Ghostman]

I'm not entirely sure what you're trying to achieve. Is the black-colored area in your example image supposed to be where the gradient would be? So there'd be gradient going from the outer edge of the area to it's inner edge?

[Thurlor]

So there'd be gradient going from the outer edge of the area to it's inner edge?

Yes, that's exactly what I want. I couldn't really show it with that image, but it's supposed to be a gradient going from black to white, with the surrounding area also black. I hope that makes sense.