Lu,
If it is not an apple, is it an orange? Pear?
It is how the rotor behaves as it flaps, it is approximated in some pretty hairy equations, and it is not generally 90 degrees.
Just because our helos are rigged that way does not make their gama exactly 90 degrees, but they are close enough so that it works. I would bet that a sizable percentage of production helos have gammas that are 10 degrees different than their controls are rigged, but it is so hard to notice, nobody cares.
When we build helos, we actually look to see what it s. I personally did it on the S-76 and Comanche. It is also not a constant for an aircraft, as slowrotor has observed, it changes with rpm (because it is a factor of the blade flapping inertia, centripital field, aerodynamic damping, hinge offset and several other things that escape me (phase of the moon?) Gamma even changes short term vs long term. If the rotor is rapidly flexed with cyclic, it dips in a different place than where it ends up long term. Boelkow drivers know this, and automatically compensate.