IMHO, I agree with plasma, you may not need to go into the service menu for proper gamma. Nonetheless, my understanding of M1, M2, etc., is a service menu gamma setting for minus 1, minus 2, etc. I believe if you set the service menu setting at M1, for example, your user menu setting of gamma @ 0 will be actually showing a gamma @-1.
I use only the user menu settings, previously using the service menu to no avail. Also, per this forum, I calibrate to get a gamma between 2.35 and 2.5 for dark room controlled light environment. Also, I set the dark tone to darkest to bring out a deeper brightness level, with the color space at auto.