To add to Krikor,
Put your image in 16 bit before adding the gradient (top menu, Image > Precision > 16 bit), or you will have banding and/or your merging gradient won't be as smooth as your example
Put your image in 16 bit before adding the gradient (top menu, Image > Precision > 16 bit), or you will have banding and/or your merging gradient won't be as smooth as your example