There are a few ways to make an image a gradient in Photoshop. One way is to use the Gradient tool. With the Gradient tool selected, click and drag on the image where you want the gradient to appear. You can also use the Gradient Editor to create a gradient.
To do this, go to Edit > Fill and select Gradient. The Properties panel will open up and you can choose the type of gradient, the colors, and where you want it to appear on the image.
Warning: This process may result in banding or other artifacts in the gradient.
The easiest way to make an image a gradient in Photoshop is to use the Gradient tool. You can also use the Gradient Editor to create a gradient, but it requires a bit more setup.