Download the PSD file
This is the easiest method I know, but it's NOT easy.
You'll need to have read these two:
http://www.damiensymonds.net/preventing-banding-in-backdrops/
http://www.damiensymonds.net/2012/10/using-photoshops-gradient-layer.html
Basically, you set up the gradient layer as accurately as possible, then duplicate it twice. The bottom two have black masks, and are set to Screen and Multiply blend modes.
You mask the top layer (using this where you can) and go fairly close to the hair. Then you start very patiently (with a very low opacity brush) you darken and lighten the difficult areas near the hair, using the Multiply and Screen layers respectively. Paint on those layers until the areas match the gradient as closely as possible.
Repeat, this is NOT easy. Allow at least an hour, maybe more.