Apply smooth blur to a part of an image?

Question

Python

Hello! can you tell me how to apply blur smoothly from top to bottom

def blur_filter(image):
    img = Image.open(os.path.join(ROOT_DIR, image))
    if img.mode != 'RGBA':
        img = img.convert('RGBA')
    width, height = img.size

    box = (0, height // 100 * 20, width, height // 100 * 60)
    img_box = img.crop(box)

    img = img.filter(ImageFilter.GaussianBlur(2))

    #imgt = Image.new('RGBA', (width, height), color=0xFFFFFF)
    #imgt.paste(img_box, box)
    #img = Image.blend(img, imgt, .1)

    img.save(os.path.join(ROOT_DIR, image.replace('/', '/copy_')), 'PNG')


if __name__ == '__main__':
    img_1 = 'img/9.jpg'
    img_2 = 'img/8.jpg'

    blur_filter(img_1)
    print('='*20)
    blur_filter(img_2)

I tried to cut out the part that needs to be filtered, but nothing happened.
I would be very grateful for your help!