How about rotating it 90º?
edit judging from your comment, it seems that you're doing this via CAGradientLayer. CAGradientLayer is a subclass of CALayer, which has a transform property. This transform property takes a CATransform3D, which is a struct that represents some sort of linear transformation to be applied to the layer (such as scaling, translation, or rotation).
So really you just need to make a rotational CATransform3D and set it as the transform property of your CAGradientLayer.
You could probably also make this work by fiddling with the startPoint and endPoint (which would actually probably be simpler).