*googles* Looks like it's pretty much the same as in GIMP: the gradient tool has a preset rainbow. So you create a new layer, choose the gradient tool, open the gradient toolbar and choose the one that already looks like a rainbow, and draw a line in the direction you want the rainbow in.

And if you have a leopard print (or any other) texture then if you fill the area you want to with that texture, then put a layer of rainbow gradient over that and adjust the opacity level until you're happy with it... rainbow leopard print!