User blog comment:B214/Volume 21 info & spoilers/@comment-83.22.168.173-20160711150248/@comment-26345952-20160820235840

I do think that G stands for God as not only he had, at least before the tune up, borrowed the power of the Dragon God of Infinity (Ophis) along the borrowed flesh of the True Red Dragon God Emperor (Great Red).

In which I also believe the reference sound of [DxD] every time boost his power in that state, just like the L for Valis DxD in reference of his power from Lucifer.