sounds like a carbon copy of the old civ 5 gold per turn exploits. they sort of fixed that problem by only allowing gold per turn trades between civs that have a declaration of friendship (or whatever it was called). technically you could still do it, but attacking a friend sort of flagged you as a diplomatic pariah for the rest of the game. also, in civ you could only trade for resources, gold and cities (cities were usually super expensive, unless some crappy village the other civ had just conquered from someone else), but not for units or techs.

not really sure if a similar "fix" would work for GC. i guess it would help if the GC3 AI only traded stuff for GPT while you had a non aggression pact with them or something. i think that would be reasonable. you could still to "lump sum" trades with neutrals, but "money per turn" sort of require some mutual trust.