Does the gradient look better when you zoom in to 100%?
Your display system, from video card (including calibration tables) to LCD panel, works in 8 bit color depth, so you never have more than 256 discrete levels from black to white. If you're working with 16 bit data and you see banding, that's generally where it happens (unless you have a 10-bit capable display system).
There are a few special circumstances to consider though. Here's a checklist:
- A monitor profile has 16 bit precision, but a defective one can cause banding.
- Photoshop previews at less than 66.67% zoom are 8 bit, but the underlying data are calculated in full bit depth, so this banding is always regular. View at 100% as mentioned above.
- Selections are always 8 bit even in a 16 bit file. A mask, however, is the bit depth of the document. But this means that a selection-based mask, used in an extreme tonal adjustment, can cause banding.
- Lightroom uses output dithering to conceal any display banding - or so I've been told. I'd like to see some documentation, though...
- With GPU enabled at "normal" or "advanced", the conversion into the monitor profile is performed by the video driver. Buggy video drivers can cause banding and other problems. In the "basic" setting it's handled by the CPU in the traditional way.
- Finally, a pretty obscure but long-standing bug/imprecision in OpenGL code. This affects ProPhoto files only, and only with GPU set to normal or advanced as per above. This causes cyan/red shadow banding to various degrees depending on the type of monitor profile. But it was originally reported with sRGB as monitor profile, so there's no doubt it's real. Apparently the conversion from ProPhoto to monitor, performed by OpenGL code, is inaccurate. The likely reason for that is that ProPhoto is highly compressed in the shadows, causing minor inaccuracies to amplify. Note that this is outside Adobe's control.
I haven't seen this particular problem in a while, but I don't use ProPhoto more than I have to (which isn't a lot).