Gradient tool, shape "radial"
layer modes
Europe.xcf.7z (Size: 478.42 KB / Downloads: 149)
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

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)