Not sure how you are creating your original gradient, but you could try selecting your image and applying a Gaussian Blur to it. That will often smooth out the effects of banding.
Also, you can try creating you gradient in Illustrator as an object, and the drag it into Photoshop as a smart object. Illustrator seems to have better support for gradients, possibly because it's handling them as vector objects rather than raster objects.
Finally, if none of the above helps, try setting your Photoshop document to 16 bits/channel and start your process over. Then when you are done flatten your image and reduce back down to 8 bits/channel.