How does the photoshopped white bottom fight
White background? If you don't understand what your white base is, do you want the whole canvas to be white or white?
Make your selection, then use a white transparent gradient and then set the transparency
If is the canvas you can create a new layer fill white, if it is suggested to use a magic wand or a lampshade box to select the area with a pen Then a new layer fill color you want
