There are several ways to do it.  You can get hold of Ajax's free shaders in the Freestuff section, which contains several gradient backgrounds.  All you would need to do is alter the colours to suit.

Or, you can load in a background prop, such as the free Infinity Cove at Runtime DNA www.runtimedna.com.  The prop will take textures, so you can create your own if a suitable one isn't available.

You could also load a square from the Props library and apply a gradient to it.

Simplest method is to render without any background, then export your image as a PNG or TIF - I always use PNG but that's just my preference - drop the image into Photoshop, then apply a gradient behind your rendered image.

If you're not already saving your renders as TIF or PNG, you really should.  They are both lossless formats and support transparent Alpha channels, which makes it easy to composite renders, add in backgrounds and generally play around with.

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