1. First make a new dokument (ctrl+n), or open the dokument where (alt+ctrl+o) you want the rounded rectange.

2. Make a rectangle where you want the rounded rectangle.
3. Switch over to the channels and make a new channel.

4. And in that channel fill the rectange with whatever you want becouse it dont mather, and deselect (ctrl+d).
5. Then go filter-blur-gaussian blur. Blur how much you ever want, but try dont to blur to much.

6. Go image-adjust-levels (Ctrl+L) and move the two naps to the middle.

7. Control-click (press ctrl+click) on the channel, switch over to layers and you got your selection.

8. Fill your selection, deselect (ctrl+d) and you got your rounded rectangle.
Greetings
C6464