Changing the force 32-bit rendering setting doesn't change anything, nor did it change anything in GB. So I don't think it's a 32-bit vs 16-bit issue. After all, we're not dealing with banding here.
Also, since the gamma test picture is rendered with each box different than the next, this is proof that it is rendered in 32-bit. 32-bit color is 8-bits per channel, which means 256 different levels of gray. 16-bit color has 4-bits per channel, or 16 levels of gray. If the test image was rendered in 16-bit, then boxes 1-4, 5-8, etc, would look identical. Since they're all unique, I think we can conclude that it is rendered in 32-bit.
So it's a completely different reason why QuickPic works and stock gallery doesn't.
Edit: Yup, just confirmed this by setting my computer to 16-bit color.