Apply gradient mask in image (with transparency)

Question

I would like to apply gradient mask into png image with transparency in python openCv.

So I have an image like here (with transparency png):

and I generated mask:

And I would like to have, the object with smooth mask gradient like did in photoshop:

I tried to do it with:

def applyGradientToReflection(reflectionPath, gradientMask):
    reflection_img = cv.imread(reflectionPath, cv.IMREAD_UNCHANGED)
    gradientMask = cv.cvtColor(gradientMask, cv.COLOR_BGRA2GRAY)
    gradientReflectionMask = cv.bitwise_and(reflection_img, reflection_img, mask=gradientMask)
    return gradientReflectionMask

but with bitwise_and it's binary without nice, smooth transition.