There is nothing magical about a gamma of 1. Gamma is the slope of the linear region of the characteristic curve when the exposure is plotted on a log base 2 scale versus density, which is a log base 10 scale of the fraction of light transmitted. The numerical value of the slope depends on the bases of the two logarithmic scales, which are arbitrary: 2 because we like to think in terms of doublings and 10 because that's how many fingers most of us have.
David