I'm not sure about the magenta ones, but the green giants are likely to stick around since FDev added a visitor beacon to the one within the bubble region.
As others have noted, all of the ones that have been found are somewhere within the bubble<->Colonia corridor. It's too small of a sample size to draw any conclusions, but it makes me suspect a few different things:
1. They tend to show up along the Colonia road simply because it's more well travelled, and no one has had the luck to find them elsewhere (and report it to the community).
2. They might be hand-placed, and exist only within that region.
I'm seriously hoping it's more #1, rather than #2. However it does make me wonder if we have a better chance of finding them by simply zipping up and down the Colonia highway.
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.