It doesn't matter what letter designation GM gave it. It's just an abstract designation to group cars with the same platform together. You know what they say about roses.
Yes, there was a "G" body vehicle from the early 70s. The 70-72 Monte Carlo. Apparently it was a longer wheelbase chevelle, and apparently there were some Grand Prixs around then that were based off the same frame too. It also has exactly zero to do with the "G" bodies we are referring to. If you want to get technical there was also another g-body platform from 95-03 (olds aurora etc...). It also has absolutely nothing to do with the g-bodies we talk about.
When talking "g-bodies" on a site that deals with 78-88 model year cars, we all know what cars we're talking about. These cars are 78-88 A/G bodies.
Yes, technically in the years '78-'81 they were A-bodies. In 1982 the new FWD versions of our cars were introduced, and then the FWD cars were called A-bodies, and the RWD cars were then called G Specials, and the 78-81 cars were then retroactively referred to as A-specials.
So they are calling the same car two different things (a-body vs. a-special) depending on what year you are in. (That is, what period of history you are in, if you were a person living in 1978-1981, the RWD cars were "A-bodies". If you were a person living in 1982+, the RWD 1982-1988 cars were "G-specials" and the 1978-1981 RWD cars were "A-specials" and the 1982+ FWD cars were "A-bodies". And some FWD cars under similar nameplates were N-bodies(Cutlass Calais for example, smaller car), and there was another 1988 Cutlass Supreme too... the W-body Cutlass Supreme, aka GM10 platform, FWD mid-size.
In the end what does it all mean? Nothing. "G-bodies" are 78-88 A/G cars. Just say that, and everyone will know what you mean. If you want to get technical, just call a 78-81 RWD car an A-body and a 82-88 RWD car a G-body. Which really doesn't tell the whole story, because the sheet metal change on RWD cars takes place in 81 (the sheet metal makes a big change from 78-80 cars to 81-88 cars, which is why you will see body panels grouped in those years, as opposed to 78-81 (a-body/a-special) and 82-88 (g-special).
Confused yet? This is why people just call them 78-88 A/G cars. Then everyone knows wtf is going on. Forget about it, and forget about the FWD POS's and the G-body from the early 70s too (cause most people call them A-bodies anyways, since they share the vast majority of their mechanical parts with the chevelles anyhow).
GM had it's head straight up its *ss when naming cars, actual name plates (cutlass, calais, supreme, whatever), or "platform designations" (A, G, A-special, G-special, W, N, whatever). For example, there were 4 completely different Cutlasses in 1988.
Cutlass Supreme Classic (RWD, G-body, G-special if you want to be an idiot)
Cutlass Ciera (FWD, A-body)
Cutlass Calais (FWD N-body)
Cutlass Supreme (FWD, W-body aka GM10 platform)
Maybe more? Who knows!? Who cares!?!?
Summary:
78-88 A/G is really all you need to know.
78-80 cars had similar sheet metal within models (Cutlass, Regal etc... some differences, ie. 1980 Cutlasses had diff header panels: 4 headlight vs. 2 headlight).
81-88 cars had similar sheet metal within models (Cutlass, Regal etc... some differences, ie. 87-88 Cutlasses had different header panels: "euro clip")
The sheet metal changes significantly between 78-80 cars and 81-88 cars.
78-81 RWD are technically a-specials.
82-88 RWD are technically g-specials.
Lord knows there are probably exceptions here and there. I, for one, don't care!