Well, you could go through each item (buildings, specialists, etc.) that give GPP and take them down a couple notches, but I would imagine that there's also some kind of threshold value in the global defines XML that would increase the number of GPPs needed to create a GP that would not require changes to all the code. Open up the file and do a search for "GP" or "GREAT_PERSON" or "GREAT_PEOPLE" or something like that. See if anything turns up...