Someone may have already posted something like this already but I haven't read through the entire topic.
However, I would like to point out that the description of the original game's GP threshold is not how it actually is.
If we go by what you said in the first post:
City 1 makes 10 GP pts per turn.
City 2 makes 3 GP per turn.
GP threshold is 100.
So yes on turn 10, city 1 gets a GP.
On turn 30 City 1 also gets a GP. (20 turns later)
However, the next limit is actually 300, not 400. (It keeps increasing by 100)
By turn 30 City 1 will have 0/300 points as it just got another GP.
However, City 2 will have 90/300 points. It doesn't get reset.
However, your analogy is still effective as City 2 in this case will not be able to get to 300 points before city 1. But it will eventually get a GP before city one. Some hundred or so turns later.
Example: Turn 60 will be the next GP generated by city 1. Making the GP threshold go to 400. This will make City 2 have 180/400 GP
Then, it will take 40 more turns, (or turn 100) for city 1 to generate another GP, making the threshold go to 500.
40*3= 120 + 180 = 300/500. (City 2's GP threshold at turn 100)
So with this in mind the next 50 turns (turn 150) will be needed for City 1 to generate another GP. 50 * 3 = 150 + 300 = 450/600 (City 2's GP threshold at turn 150.)
Then, 60 turns later again, (or turn 210) technically City 1 would get another GP at 600, however, it will only take 50 turns for City 2 to get it's GP. (600 - 450 = 150/3) = 50
So, as you can see it isn't impossible for a city less generating GP pts than another to end up generating a GP in the original game, it just takes a bit longer.
Regardless of that long speech.
I still think this is an awesome mod.