AFAIK, the only real inefficiency of a GPT-deal is the one that Firaxis deliberately programmed, i.e. the [gold-]total acceptable to the AI will always work out higher for a 20-turn GPT-deal — by about 10–15%, as far as I can tell (rough estimate!) — than for a one-time up-front payment. But provided that your own towns/economy continue to grow over that 20 turns, those "interest-payments" aren't generally problematic (especially if you manage to goad the AI into breaking the deal early!)It also appears to work for gpt, though gpt deals have some inefficiencies in many cases.
And if you took off the lump-sum and clicked "Per turn" instead, he'll accept 2 GPT (40 gold total), because 1 GPT won't give him the 29(+) that he wants. But if you offer him 1 GPT and then click "Lump sum", his up-front payment-request will likely go down to 9–10 gold.In my example, I ring up the Maya and offer them Philosophy for Mathematics and The Wheel. Smoke will probably like it, so I then click lump sum and I get an autofill of 29. That's the exact optimum number for that deal.
That's one of the things I really like about this patch: based on what I have in my Treasury, and how much income I'm getting, I can very quickly zero in on an optimal deal for my current situation.