Here are some gradient filling options I wished where integrated in the Gimp :

1) 4 way gradient : Each pixel of the selection has a distance computed as the 
shotest number of pixels beetween the starting point of the gradient and the 
pixel by only walking horizontally or vertically. Min value for the starting 
point, max value of the gradient for the distance of the destination point.
This would look like the square gradient, but with a dimond shape, but would 
follow the shape of the selection. I can make example images if I'm not making 
myself clear.

2) 8 way gradient : same as above but with diagonals included as authorized 
walking steps. This would look like the square gradient, but with shape 
following.

3) Real distance gradient.
Trickiest to implement, the distance is computes as the real shortest pixel 
distance with the ability to walk in any direction but not to leave the 
selected area.
This would look like the radial gradient but following the shape.
For example, you could select a non overlapping shape of a snake, then apply to 
it a gradient starting from it's tail and ending at it's head and the gradient 
would follow the complex sinuous shape of the snake smoothly.