There is no gamma when in HDR, instead a similar process is EOTF. In the case of the HT5500, there are five settings choices: -2, -1, 0, +1, +2
I mention many times in this review that the HDR picture of this BenQ tends toward bright. The middle setting 0, definitely is that, so +1, and +2, were way over the top for me.
This photo player has the same image, same exposure, for 0, -1, and -2. You'll notice there is a rather substantial difference between them. Personally, I mostly prefer -1, but I've used -2 almost as much. I have used 0 as well, but often when the room isn't fully darkened. You may like it brighter than I do.
Bright scenes show every bit as much difference between the settings.
Even at -2 I find the BenQ a bit more bright (but just a little) in the upper ranges, than several other very good 4K projectors.