Color map-indexed image

[chakravartysourav]

I am looking to stretch out my 256 color positions in the indexed color map using two colors. I can select the individual boxes and change the color, however, I was hoping to select all boxes at once and define the darkest and lightest box colors and then let gimp spread out the color across the boxes. In photoshop I can just select the first box and then drag until the last, is there any similar option? 
Attached image from photoshop

[rich2005]

Clever old PS never seen that one before

Using Gimp a bit more hands on. First thoughts, a couple of ways

Make a gradient using the FG / BG gradient. Once selected those colours appear as a 'custom gradient' top of the gradients dock
A right click in the Palettes Dock and import a new palette using that gradient as a source.
Map > Set Colormap using that new palette.

https://i.imgur.com/NR9NzuY.mp4

or for a quick-n-dirty way using indexed images inherit the colormap of the original image
Make a new blank image
Fill it with a gradient
Convert to indexed.
Click and drag the original image over as a new layer.

https://i.imgur.com/yRLrGTy.mp4

Note the various modes for gradients, it does make a difference for colour distribution.