What was done:

1. Create a new image.
2. Change mode to grayscale.
3. Select a black and white linear gradient.
4. Cover the canvas with that gradient.
   (See 'before_gradient_map.png' to see my canvas at this stage.)
5. Apply gradient map.
   (See 'after_gradient_map.png' to see my canvas at this stage.)

What was observed:

   The gradient is split. Rather than a gradient from black to white, the canvas shows a gradient from 50% gray to white, then black to 50% gray.

What was expected:

   Mapping a black and white linear gradient to a black and white linear gradient shouldn't change anything. The behaviour appears to be correct in RGB mode.