1) Create a document in 16bit NOT 8bit, or just go to Image -> Mode -> 16bit
2) Create your gradient
3) Merge your background layer with a brand new layer so that it's a full layer without any blending options added (or just rasterize it).
4) Go to filter -> noise -> add noise
5) Tick the "monochromatic" box.
6) From there, select the percentage as 0.5% and then hit ok or apply or whatever.
7) If you can still see the colour banding MAJORILY, then change the percentage to 1% or higher, but remember not to add it too high.
8) You're done, colour banding is fixed.