No, I think we can rule out number 2. GGG 5 was reported far away from the Colonia route, and was reported before Colonia was founded. I'm fairly sure it's a quirk of the procedural generator. The bias towards the Colonia Road is simply because that road is more well-travelled.
Adding to the bias, I would assume (!), is the likely star types they can form in: they can form in pretty much any star type, just like regular gas giants. If one forms, it's likely to form in an M or K system since those are more probable overall - and deep-space explorers tend to filter those stars out as they're less likely to contain ELWs and other valuables. The Colonia Road is more likely to have Ms and Ks scanned because that route is popular with the Bubble-Colonia commuters and tourists who are travelling, rather than exploring.
GMGs, on the other hand, are like GWGs were: if they're limited to Class V, then they'll only be found in systems where Class Vs can occur: super-hot stars, A-class and above. So it's more likely that deep-space explorers will spot a GMG, even if they do turn out to be much rarer than GGGs.
My own theory on the generation of GGGs (and by extension GMGs) is this: the stellar forge randomly generates "sets" of colours for each gas giant, two or three colours depending on the giant type. The generator also seems to pick "matching" and "contrasting" colours, also depending on the planet type. I've noticed that many GGGs have black as one of their colours. I therefore presume and suspect that the stellar forge, in trying to pick a "matching" colour for black, is trying to go "blacker than black", going negative, and therefore ending up bright green. For GMGs, perhaps it's "contrasting colour" calculator is trying to do likewise, in calculating a contrast for white, it tries to go blacker than black and crosses over into magenta.