Simple explanation - this crazy tinker named String theory build a gun, the F-Driver (Firmament Driver), powerful enough that if she shot the moon, it would fuck up it's orbit. The G-Driver is the upgraded version of the thing, and powerful enough to hurt Scion. The other things on that for brute force attack are um, Lung with 2 years of charging up.
In exalted terms - imagine a weapon second only to the godspear and other infinite damage weapons and combos. Now imagine it can be mass-produced, has functionally infinite range (at least in creation), and then imagine someone upgrading it to make it even nastier. That's the G-Driver.