This is the result of having multiple ways of boosting gp points at the same time in the same place. Any time you have multiple items that can provide bonuses to the same thing
and can
potentially all be acquired at the same time, this kind of thing can happen. The game is usually balanced with the idea that one or two might be stacked, so having more than that can cause 'problems'.
The problem is that nerfing the individual components makes having just one of them virtually useless. Eliminating some of them means you have less options for how to acquire such a bonus. Neither is a very good approach I think in our case.
The other less taken approach is to use a 'diminishing marginal returns' approach. If you've taken an economics course at any point in your life you've heard this term, wether you remember it or not. To put it simplistically, it's the basic idea that the more you invest in any one aspect, the less incremental return you get for each unit of investment.
In game terms, that would mean a city with a base 25% bonus might receive that full bonus. However, if you push it up to a base 50%, the actual might only be 40%, @ base = 100%, actual = 65%, @ base 150%, actual = 75%, etc. (purely made up numbers here for an example, it can follow whatever mathematical model best fits the situation)
This works well for internal numbers the player never sees (I've used just recently for espionage defense's effects on capturing nearby spies). However for numbers that will be publicly displayed in the interface, it can create confusion for players of where the numbers are coming from.
When 1 + 1 no longer equals 2, people get uncomfortable
.
So every approach has it's own drawbacks to consider. At present the main (and probably best) limiter is the opportunity cost of building the buildings and wonders that boost it, or taking the civic (as opposed to some other useful civic). At higher difficulty levels, it's
generally harder to amass all of them, and a bunch of wonders, and/or specialists all in one city to do this. But it can be fun when you do manage it
.