(And I have trouble accepting the suggestion that Nod can somehow hide all evidence of operations on that sort of scale. Especially agriculture, which takes up lots of space and needs to be above ground to avoid ruinous power costs, but putting all the rest of it underground or under extraordinarily sophisticated and difficult to construct stealth technology also sounds non-viable. Nod's assets and population base just have to be too big for them to be a significant threat to GDI to hide.)
GDI has extensive hydroponic and aquaponic vertical farms powered by nuclear reactors because tiberium infection is too great a risk for open field farming.
Nod, living in the more heavily contaminated YZs, no doubt has similar practices. In fact, during TW2, key civilian infrastructure in tiberium infested regions were things like water purifiers and enclosed hydroponic farms.
What's the estimated world population, and the portion of it behind GDI's lines?
Somewhere in the ballpark of 2 billion worldwide, I think, and around 30-40% are behind GDI's lines. I found a WOG on discord that we're not likely to hit 3 billion people world during the quest.
At the end of TW3, world population was about 1.5 billion. A third of that was GDI.
These days it's probably about the same number of people, maybe having crept up to 1.6-7 billion and it's more like 40-45% GDI.
Population numbers are at the lower end of between 1.4 and 1.5 billion people, of which some 550 million live in GDI territory. Nod has a shrinking population advantage, but that's because Nod's population is dying rapidly and fleeing its territory. It retains, however, have a notable advantage is military age population, because their population is on the average younger.
The population of humanity will not, under any circumstance, rise except if any of the following happens; we manage to establish substantial space habitation, we reclaim substantial portions of the world from tiberium even through its current mutating phase, or we build the TCN. People are too despairing to have more kids, basically, and humanity as a whole is not achieving replacement rate. Nor is Nod, specifically, and forget GDI.