That’s interesting. I wonder which publisher(s) use that terminology. I don’t have a lot of guitar books in the house, but the few that I do just show “G” for 320033. G5 just shows up as the usual power chord 35(5)xxx or the 3x0033 chord. Must be some oddball quirk of a certain publisher or something. (Even online, I can’t find a G5 spelled 320033.)