Another reason to early-gift a city to a distant AI is for espionage tech stealing. Before they have 4 cities you can gift a city near your capital, with your culture in it, re-capture when appropriate and you'll get max discount techs later on. The AI will always accept it as a gift after having owned it, so you can gift it with a fully fortified spy, steal, re-capture. Rinse and repeat. Not so good with Creative AIs, though because you don't get the full 50% discount for your culture in it.