Best way? Go gradually from 100 to 36% opacity

[rich2005]

A bit pushed for time, it being the Friday meeting (for a couple of beers)

I might go this way.

1. The brain layer at reduced opacity

   

2. The brain layer duplicated, Opacity up to 100, a layer mask (white) added

   

3. A radial gradient, black-to-white applied. Center is solid / edge is reduced

   

Edit:

A better possibility, same as before but with a different gradient option.
First make a selection of the brain Maybe Layer -> Transparency -> Alpha to Selection
Then in the layer mask, a shaped gradient which follows the selection boundary Then adjust the mid-point.