The idea of adventure modules "codes" -- D1, G2, Q1, etc. -- is well established in the history of TSR era D&D, so much so in fact that a lot of latter day old school publishers continue the practice. Gamma World adopted this practice (as did, I think, all of TSR's RPGs), but it also introduced, at least in its first two modules, a further identifier for its modules. Take a look at this section of the cover of 1981's Legion of Gold:
Famine in Far-Go:
I don't know what to make of this. My guess is that, even in 1981 and 1982, TSR was still struggling to make sense of both their own success and this hobby they'd helped to launch. They were trying to find ways to market their products and ensure their utility to consumers. So, codes and types experiments of sorts, only one of which continued to be used in the long term. I imagine, if we were to look closely at the adventure modules published for other TSR games, we might find similar experiments in labeling and identification that didn't stand the test of time.