8 bits per channel, 24 bit total (8 bits for Red, Green, Blue channels). In subtle gradients even 24 bit images can still show banding. The problem here is the image is being stretched in your game engine and, being fairly low resolution to begin with, you see the flaws much more easily. Make the JPG export as high quality as possible, or if your game engine supports it, use some other image format with lossless compression or no compression.
In TG, if you're seeing the banding in direct BMP output, check to make sure you haven't reduced Sample Jitter in the Atmosphere node Quality Tab below 1.
- Oshyan