In GIMP is it possible to create a gradual effect in selected area?

[dRebelXT]

For example when I select the area arround the eye to sharpen/contrast, or a part of
skin to lighten up, it might cause a steep transition with surrounding.

How do I make the tool to give 100% effect in the middle while 0% in the edge of my
selection? In Photoshop is it possible also?

 

[scandisk]

For example when I select the area arround the eye to sharpen/contrast, or a part of
skin to lighten up, it might cause a steep transition with surrounding.

How do I make the tool to give 100% effect in the middle while 0% in the edge of my
selection? In Photoshop is it possible also?

how about trying feathering on the selected area ?