1. After opening a New window (go to File > New and OK) and a New Layer by going to Layer > New > Layer and OK.
2. Stamp some random brush(es) with Layer 1 highlighted. I am going to use the brush below:
3. Now, with Layer 1 highlighted, go to Layer > Layer Style > Gradient Overlay… like shown below and pick the gradient you want and press OK.
4. Finally, you have your gradient ready. –Karina