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.