The only requirement is the 'gift' city be within 9.5 plots of that AI Civ's nearest city.
There are several posts in past SGOTM team threads that make it clear that this is the only requirement. My recent game play confirms this to my satisfaction.
EDIT: Sorry, there are financial requirements that the AI must meet as well. Located in SGOTM team threads. Sorry, but I can't provide a specific link at the moment.
孫子武