Just use in the CSS whatever you would use in a fill
attribute.
Of course, this requires that you have defined the linear gradient somewhere in your SVG.
Here is a complete example:
rect {_x000D_
cursor: pointer;_x000D_
shape-rendering: crispEdges;_x000D_
fill: url(#MyGradient);_x000D_
}
_x000D_
<svg width="100" height="50" version="1.1" xmlns="http://www.w3.org/2000/svg">_x000D_
<style type="text/css">_x000D_
rect{fill:url(#MyGradient)}_x000D_
</style>_x000D_
<defs>_x000D_
<linearGradient id="MyGradient">_x000D_
<stop offset="5%" stop-color="#F60" />_x000D_
<stop offset="95%" stop-color="#FF6" />_x000D_
</linearGradient>_x000D_
</defs>_x000D_
_x000D_
<rect width="100" height="50"/>_x000D_
</svg>
_x000D_