Foreground to transparent gradient filling to solid foreground color

I am trying to do a simple white to transparent fade over a grayscale photo (so that the edge fades to white). I picked the preset Foreground to Transparent with white as my foreground color, but it fills the entire window with solid white. I've tried with different foreground colors, tried using the legacy preset, tried doing a black to transparent mask, and nothing seems to work. The gradient tool always fills the entire window with a solid color. This used to be a 2-second maneuver but it's taken me all morning so far...