How can i create a mask of grays?

Hi

i did learn how create a saturation mask (a mask that contains the colors)

how can i create a mask with the grays ?

can i do it? how can  i create it ?

the only way i can thing it's use a color selective adjustament layer and set to +100 black every sliders outside grays

but i don't think it's a mask with all the Grays

thanks