I am going to explain what I think I know and how that leads up to my question - please correct any false claims I make or conclusions I draw.
I initially thought that $G$s were defined purely by acceleration. This confused me though, because when a body is at the surface of the Earth, even without change in velocity, it is said to be experiencing 1g. Furthermore, when in free fall, a body is said to experience 0g.
One answer I got on Quora said that $G$s are actually defined by the resistance to acceleration. This seems to make sense, when in freefall I am not resisting any acceleration, and when I am standing on earth I am resisting 1g.
However, this started to confuse me when I began to add other forces. For example a body in a complete vacuum is acted on by a force. The only resistance to this force is the body's inertia, which resists acceleration. That makes sense to me. However, what happens when I add resistive forces? Do you only take the resultant force? That seems an attractive option, because the resultant force is the only part of the force that induces acceleration. But, if I go back to earth, when I stand on its surface, I have gravity pulling me down, and I have the force of the earth's surface (normal) resisting my motion. If I am not accelerating, that means there is no resultant force, and yet I still experience 1g.