...firstly the image is actually using a white (with low opacity) as opposed to a grey...
...there are a number of ways to go about this, the simplest is by way of creating a selection with the marquee tool (M key) where you want, create a new layer above your base image, then still with the selection active choose the gradient tool (G key)...
...in the tool bar you have the option to change the gradient, make sure you have it in linear mode, click on the gradient proxy preview top left to change the gradient to a white > transparent gradient...
...with the shift key held down, drag from left to right over the marquee selection...
...in the layers palette, adjust the opacity of the gradient layer so the white become lighter and thus showing more of the photo underneath...
...the other way of doing it is to use a layer mask on a white filled selection...
Andrew