1. Open GIMP. After Opening Gimp , make a new canvas.
2 . Make your texture (I am gonna use a basic one.).
3 . Color it as you like.
4. Make a new layer , color the edges black . And Make another layer and color the inside white. Like This :
5. Gaussian Blur both of the layers with 15 (Blur Radius)
6. Adjust the opacity and your done!