You can’t make any single color darker than itself at 100%,
In order to pull off a gradient, you would need to use a tint of the color for lighter areas. You could create a gradient of something like 100%-80%-50-100%. Where 100% is the solid color. However, that is the darkest it will get.
This is easier in Illustrator or Indesign…
For Photoshop you would need a Spot Color Channel they simply apply a white gradient over black on that channel.
Results of tinting a metallic spot color will, most likely, not be what you expect. You may want to discuss tinting metallics with the print provider.
Your sample is mixing black with the color, which you can’t do for a single spot color
