Gradient tool, shape "radial"
layer modes
Europe.xcf.7z (Size: 478.42 KB / Downloads: 97)
Screenshot below and .xcf above, should be self explanatory, increase contrast for a sharper effect, check out the compressed .xcf for the different layer modes (Layer with the mask is in "Darken only" mode)
layer modes
![7z archive .7z](https://www.gimp-forum.net/images/attachtypes/7z.png)
Screenshot below and .xcf above, should be self explanatory, increase contrast for a sharper effect, check out the compressed .xcf for the different layer modes (Layer with the mask is in "Darken only" mode)