I was really pissed off when I found out that the increment between successive GP increases to 300 after you get your 20th GP (e.g. 3000, 3300, 3600, ...), as darrelljs notes. That means it would take:
30th GP: 6000 GPP (6400, 6800, ..., 9600)
40th: 10000 (10500, 11000, ..., 14500)
50th: 15000 (15600, 16200, ..., 20400)
60th: 21000 (21700, 22400, ..., 27300)
70th: 28000 (28800, 29600, ..., 35200)
80th: 36000
etc.
So, Xanadux, as for your plan to play settler and have 30 cities with 20 specialists each by turn 300 -- phil/pacifist gives 180 GPP/turn per city. With 200 turns left, that's 36000 GPP/city, or 1,080,000 GPP total -- larger than the sum total amount required to get to 80 GP (1,038,000, as darrelljs says). So you've got the required amount of cumulative GPP ... but not by a large margin, and it would be distributionally way off. I think you'd need more than 30 cities ... I'm trying a sim in Excel right now.
UPDATE/EDIT: Xanadux, after running a sim, I think your plan would leave you about 10 GP short, even with some liberal assumptions about GP generation prior to turn 300. And getting it right would require a lot of depth of vision, as well as a sizeable amount of micromanagement of GPP generation ... But give it a shot. It could be an interesting challenge: settler, large rainforest map, time victory only.