What is G force? It is the load factor, or nz. What is the load factor? It is acceleration. Referencing it to the normal acceleration on the surface of the earth is just a way of making the number convenient.
It is effectively a unit used to convert the g force into a quota we can easily relate to, rather than a hard-to-grasp number. "4G" tells us more than "40 m/s^2", even if it is the same data.
The question was "how to calculate the g force", not "how to transfer the g force into a convenient yardstick".
Compare it with how
speed = distance/time
is how you calculate the speed of a train.
Dividing the acceleration by the standard acceleration is the equivalent of dividing the speed by the freeway speed limit, in order to give an easier number. "It goes three times as fast as a car on the freeway".
G-force - definition of G-force by the Free Online Dictionary, Thesaurus and Encyclopedia.
"G-force
A force acting on a body as a result of acceleration or gravity, informally described in units of acceleration equal to one g. For example, a 12 pound object undergoing a g-force of 2g experiences 24 pounds of force."
My printed aviation dictionaries are in boxes at the moment.
Cheers,
Fred