Hi Rude Dog I'm at about the 42nd lat. The reason GG is so early is because it's a semi auto. Which is just fancy wording for easily triggered flowering. Hope this helps.
Latitude definitely plays a role in bud development for semi auto or any strain really besides full auto's of course. It's all about when the photo period changes, how rapidly, whether or not the plant is root bound ect. What lat is your grow? GG may be a good choice next year. Lats closer to the equator and further north start bugging sooner or maybe that's backwards? Not sure...
Not all semi auto's flower at the same time that should be taken into consideration as well. Even within the GG lines. For instance my GG2's look to be at about the same stage as your marco. Maybe even a little behind. If you kept a male or collected pollen that earlier pheno would be a great breeding plant. Especially if you had a early male to.
GG's were breed at lat 52 greens stated that he planted around the end of may to beginning of June from seed and would harvest from early August (autoed GG2's?) on into mid/late September (GG crosses).
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On a side note: I may have to harvest tomorrow or when ever this hurricane is over. The winds are so intense large trees are toppling over, some plants if not all GG2&3 have with out a doubt broken. Since they are in a more open area then BDxGG's.
Ps. Your plants are huge! Congrats going to be a great harvest.