Chieftain to Regent: "Lord Emsworth Deals". However, for the GOTM these are banned as an exploit. (For the HOF, they are allowed, iirc. Spoonwood will be able to confirm this.)
The type of "lord emsworth deals" that Lord Emsworth did on Chieftain are not legal in the HoF. The example you gave also would
not be legal for the HoF (see "giving the AI more gpt..." section at the bottom of the page). They both put free money into the international economy. That's specifically disallowed.
What does end up legal though lies in setting up a trade route, trading for luxuries (or resources) + hard goods with gpt, and then disconnecting your trade route IF you only take gpt that the AI has. The AI will take the reputation hit, since they have the responsibility to supply resources under all conditions, and need a trade route. I have said that could described as a type of Lord Emsworth deal, since he advocated for such. BUT, you can only take whatever the AI has. If you double dip, that's a slip, and you've broken the rules.
As
another example of what would be illegal, easier to setup than military alliances and selling techs, you can't, according to HoF rules (and XOTM competitions), pay something like 100 gpt for spices to an AI with no surplus gpt available for trade (legal so far). Then the AI has all those spices and may have 100 surplus gpt. So, the next part of a deal like Lord Emsworth was doing would have the AI pay that 100 gpt for your lump sum of gold (legal so far... the amount of gold you need depends on AI aggression level). Then you trade more gpt for say wines and that gold (legal). Then you disconnect your trade route (now it's illegal!). The illegality slips in, because you end up accepting more gpt than the AI can afford. It puts free money into the
international economy.
Now, again you COULD do something like pay 1800 gold for 100 gpt from an AI that has surplus gold WITH no gpt going to it from you. Then pay 200 gpt for wines + that 1800 gold from the AI. Then disconnect your trade network. That's perfectly legal. But, will Chieftain to Regent AIs have any gpt available without you supplying any to them? Yea, good luck with that. Additionally, you end up having fewer roads, or needing to pay massive numbers of workers unit support, so much so that just having roads everywhere ends up likely more profitable Chieftain to Regent. Now, at Sid, for sure, not having roads in 8 tiles, and a half dozen or so workers to road a tile next to your capital is much less than what you can get from the AIs, once they have gpt. Probably also at Demigod and Deity (though selling tech that you research for gpt is much more viable at this level... and waiting for AIs to research technologies may slow down the tech pace). But, at level between Chieftain and Regent such a scheme would end up a waste of time.