Deal or No Deal
No AI is gonna pay us GPT. We can get 10 gold from an AI now and then, but it won't be enough to make up for the gpt we pay by far.
Indeed, they may drown themselves in units. They won't all be alone on islands though, so they can fight with the units.
Fruthermore, even if they drown themselves in units, paying gpt is no sollution, it will just cause them to have more units before being drowned and we are paying their upkeep rather than researching. The best way to avoid this i think is inciting wars. I do not know if this will cause problems for our diplomatic goals.
That is all later in the game however, at the moment, the AI seems to be researching pretty darn fine. They have 3 cities and they pay 60%, surely, they research faster than we can.
And please note, getting Lit for free is almost as good as getting currency for free even if we wouldn't pay a peny for it. And if we want to ensure India keeps researching, we should get thim into war rather than giving up our own research for them. They are not alone on their island. (of course, their enemy should be a weaker one, or there must be a third on the island for a 2v1 war.)
About the horseback riding thing:
It really has been a long time since i did this.
If we research HBR to get into the middle ages, will it be possible to gain more than just the first 3 techs from the AI's free techs ? Can you make each and every scientific civ get another tech until we are halfway trough the middle ages ?
I'd assume, to do that we need to buy the techs for gold. We would first have to gift them the available techs so that they will get another free tech, then we can buy their free tech. Then, we could do the same with the second civ. get back our gold from the first with the tech from the second and proceed to the third. So this would require the gold to buy 2 MA techs. gpt wont really work since we only get back part of it and we will quickly run out.
If you don't think we can get more than the 3 starting techs, i think we can just take HBR in trade, bring ourselves and the scientifics into MA and trade. There are enough scientifcs that we will get all 3 of those MA techs amongst them to trade.
If the AI's are really gonna drown themselves into upkeep, maybe we should kill none of the scientific civs and do that tech cascading thing and buying them with gold when we go into the industrial age. For that, we would need to delay going into industrial and first stash loads of gold, but if we can then get one free tech for each scientific civ, it will be more than worth it.
We cannot likely stash gold before middle ages or the AI's will have done their own trading tricks before we get there.
BTW, if i am gonna be outvoted and you happen to take the deal anyway

, i think the only way to get back a significant part of the gpt is by doing it immeadiately. If you wait a few turns, i think the gpt will be incorporated in their economy, paying their upkeeps and we may not be able to get back any of it.