I tend to play at Online speed, and I think that:
The AI think that 1 GPT is equal to roughly 14.2 Gold. Basicly, I could exchange 100 GPT for 1416 Gold.
For example, the IA want me to pay 18GPT+2 pour 20 Diplomatic Favors. I can change it to 259 Gold instead, saving more than half the Gold cost over time. Then I could sell back to 18GPT+2, then buy back to 259 Gold. Basicly, you can convert your flat Gold into GPT for a 14.2 rate.
It could be an amazing tools in the early game: without any GPT, enemies can't sustain an army without dissolving it but you allow him to speed up his early game by straight buying.
Strategic ressource have high value when they have less than 20. For example, if the AI have 8 Horse, I can sell it 12 Horse at an amazing price, but 13 get back to 1 Gold. Maybe for higher speed it is different (20 is the minimal amount for 1 unit I guess). The value is pretty linear in that case between 1 and 11 Horse, but, somehow, the IA is willing to give 50% more for exactly 12 (so it would have exactly 20).
The AI don't sell linear. 1, 2, 3, 4, and 5 Iron is costing 5, 1GPT+2 (17), 2GPT+8 (38), 4GPT+9 (67), and 7GPT+4 (105). But you can buy 5 times 1 Iron to 5 Gold. Your Iron is worth 4 Gold. Basicly you can buy 20 times 1 Iron for 100 Gold, and sell 20 Iron to 120 Gold (due to +50% increase for exactly 20) that magicly convert to 8GPT+5.