This link should help you. You will find the syntax for gradients there.
It's this one for all the major browsers:
background-color: #444444;
background-image: -webkit-gradient(linear, left top, left bottom, from(#444444), to(#999999));
background-image: -webkit-linear-gradient(top, #444444, #999999);
background-image: -moz-linear-gradient(top, #444444, #999999);
background-image: -ms-linear-gradient(top, #444444, #999999);
background-image: -o-linear-gradient(top, #444444, #999999);
background-image: linear-gradient(to bottom, #444444, #999999);
filter: progid:DXImageTransform.Microsoft.gradient(startColorStr='#444444', EndColorStr='#999999');
…while #444444 is the color at top of the gradient and #999999 the gradient-color at the bottom.
The different "vendor-prefixes" ensure that the gradient works in different browsers as the 'default'-syntax is not supported by every browser by now.
The filter-property will make the gradient work in Internet Explorer 8 and below. But this has some drawbacks (performance aso…). Just use it if really necessary.
Edit: The syntax for linear-gradient changed. The spec'd syntax:
background-image: linear-gradient(to bottom, #444444, #999999);
I've changed this above too, so everyone can just copy this.