How to create a brightness gradient?

[nosle]

The typical workflow for this type thing is:

1. Duplicate layer
2. Desaturate duplicated layer
3. Blur desaturated layer untill no features are visible
4. Invert blurred layer
5. Reduce opacity of inverted layer to 50%
6. Merge file and recover contrast using levels or curves.

At step 5 you can also experiment with layer blend modes such as overlay.