Where does the built-up SE corner fall on the diagonal?
Do you happen to know the slope of these lines?
If these lines are repeated to the SE they seem to intersect some more real estate (including G1c).
I'm currently working on trying to determine the exact nature of the structures under G1 and G2. I believe there might be two distinct structures under G1 and one might share a boundary with this grid pattern.
Perhaps a final question might be how closely the grid pattern matches the average strike between the corners of G1 and G3.