I'm late to the party, but this is (unbelievably) still a problem as of the 11.05.2017. Here is a simple solution which will also work cross-platform with linear gradients:
.backgroundFixed {_x000D_
background: linear-gradient(160deg, #2db4a8 0%, #13af3d 100%);_x000D_
background-size: 100vw 100vh;_x000D_
position: fixed;_x000D_
top: 0;_x000D_
left: 0;_x000D_
height: 100vh;_x000D_
width: 100vw;_x000D_
z-index: -1000;_x000D_
}
_x000D_
<!DOCTYPE html>_x000D_
<html>_x000D_
<head>_x000D_
<meta charset="UTF-8">_x000D_
<title>title</title>_x000D_
</head>_x000D_
<body>_x000D_
<div class="backgroundFixed"></div>_x000D_
<div class="paragraphContainer">_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
<p>We're here to make the body scroll</p>_x000D_
</div>_x000D_
</body>_x000D_
</html>
_x000D_