You can achieve what you want like this: Place your background at 0px 0px
and define a gradient with more color-stops, having one solid color area at the top and then the actual gradient, like this:
background-position: 0px 0px;
background-image: linear-gradient(to bottom,
#FFFFFF 0px, /* Have one solid white area */
#FFFFFF 255px, /* at the top (255px high). */
#C4C7C9 255px, /* Then begin the gradient at 255px */
#FFFFFF 100% /* and end it at 100% (= body's height). */
);
(The sample code works on latest versions of Chrome and FireFox, but adapting it to support all major browsers and versions is straight-forward, applying the same principle.)
See, also, this short demo.