One suggestion: whatever you do, don't use a grey background, not
even as a gradient. Use a background with some dark color in it,
either dark blue, dark green, or something. Ideally, something that
compliments the colors of the pieces, but if not, go with a dark
blue to darker blue gradient.
The reason being that grey is one of the few colors guaranteed to
highlight any miscalibrations or other color issues you might have
with your image.
Getting any device, from simple RGB monitor to multi-thousand dollar
printing press to accurately render a true neutral grey can be, and
frequently is, the sort of fun I bill out at $200/hr to solve.
There's almost always some color cast in the image, and having what
'should' be a neutral grey that clearly isn't just highlights it. So
rather than fight your greys into true grey-ness, it's easier to
make it really clear you weren't trying for a neutral grey
background, thus the use of some sort of color. I suggest dark
blues, and greens because they're on the cool side of the color
wheel, and tend to recede visually, thus highlighting your piece.
As far as gradients go, if you have Photoshop (Or almost any other
photo-editing software I can think of) you can make your own very
simply. They all have gradient building tools.
If you can't spring for the 800# Gorilla that is Photoshop, check
out Graphic Converter. I think there's a PC version of it. The last
Mac version I looked at was very impressive, and I believe it's
still shareware at some reasonable price.