Use gradient as a fill first, then select Edit and Undo, then apply
To apply the effect to:
The canvas — Click the Canvas in the Layers panel.
A selection — Click a selection tool from the toolbox, and drag in the document window to select an area.
A layer — Click a layer in the Layers panel.
Choose Effects Esoterica Pop Art Fill.
Choose Image Luminance from the Using list box.
If you want to work with an inverted version of the source, enable the Invert colors check box.
Adjust the Scale slider to set the dot size.
Adjust the Contrast slider to mix in the luminance of the control medium.
Click the Dab Color chip, and choose a dot color.
Click the Background chip, and choose a background color.