There is a select by color feature in Photoshop that will allow you to select an altitude region with the eyedropper. You could then use that selection as a source for a mask on a texture layer. I don't have PS available to me right at the moment, but I believe it's Select > Select by color... Then the mask refine tools can be used to adjust the transition zones between textures.
I don't think there's a way to directly create a texture gradient. I would think that it would get really muddy with a continuous gradient, anyway.
Also, the advantage of using the masks on texture layers is that the workflow is non-destructive and you can modify the masks separately from the heightmap, allowing you to customize the biomes instead of having them follow the altitude absolutely.