Been trying to figure this out for awhile, and google hasnt been much help.
I am using a charcaol brush on a stroke in CS6 and when I pick a gradient it just turns the color completely black. The gradient works if I make the stroke basic though. I am guessing it is not possible for whatever reason? I also tried to recolor the artwork but couldnt find any gradient options in there.
I suppose a workaround would to maybe make a duplicate of the stroke, make it a basic shape and blend the 2 or something?
Possible workarounds would depend on what you want to do with this artwork. Does it need to be editable?
If not: expand the brush stroke (Expand apperance) and fill it with a gradient
If you need to edit it, you might consider an opacity mask: Create a normal stroke with a gradient, put it behind the brush stroke and Click on "Mask" in transparency panel. Then invert the mask
You could also create a brush that contains a "gradient" simulated by a blend and then apply that to the stroke.
Here are some fading brushes you might find helpful.