How to create 100% black (k) to transparent gradient in Photoshop

Hi everyone, I have a question regarding creating 100% black to transparent gradients in Photoshop. I am trying to create a 'black plate separation'psd file that I can embed to my illustrator artwork, to show a fade from a 100% spot colour (black) to transparent. I have several spot colour gradients I need to make in this way, and I thought it would be easier to create just 1 black plate, that I can specify to the printer will print a certain Pantone.

Do I start with making a greyscale psd file and create a new gradient in the gradient editing menu? When I have tried this, the 100% black (k) changes to the cmyk default version (72, 66, 65, 74)?? I can't make the gradient stay as 100k to 0? What am I doing wrong?

Thanks, Caz