I believe it's where the antler branches off the main beam.
The G simply refers to the main typical points on the rack. When scoring a set of antlers, there is of course a specific formula that is used. The formula goes through an A, B, C, etc. method. A, for example is total number of points. B is the tip-to-tip spread. C is the greatest inside spread. And so on. On that list, G is the length of typical points. And while nontypical points are simply added together for a grand inch total, typical points are added separately... Thus, the antler points are labeled the G1, the G2, and so on. Basically, G1 is the brow tine, and G2 is the next point up the rack, and so on.
No...If you go to the link I sent you, plus TXSUT pretty much explained what the "G" stands for.