Pick the spot you want, presumably from the Pantone swatches, and add to your color swatches. Go to the triangle in in swatches window and duplicate that swatch. Keep duplicate selected and go to top of window - tint - and select 20%. Now you have 100% and 20%
Open Gradient window. Drag both of swatches - 100 & 20 - to the gradient bar and drag the original 2 colors on the gradient bar off the bar - leaving just the 2 new ones - 100 & 20. Make any changes to the new gradient as desired.
You will get some shifting. The pantone grays like warm 9 don't shift much until the numbers get higher - like 10-12. I'd look at the Pantone solid to process guide and try to use colors with the least shift. You get teh shift becuase you're blending the 2 in teh gradient, essentially creating new colors.
Using OSX 10.3.9 on a G4