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