I have only played one game so far up to mid-Industrial age with the Celts on Emperor, Epic, Large but I can already say that G&K is a great addition.

RELIGION
This is already being discussed elsewhere but the wonderful terrain specific bonuses you can get can make just about any start viable. Brilliant addition. Might require some tweaks/balance for specific combos but the basic idea is a wonderful addition.

HAPPINESS
Perhaps this was the intention but I have been running 30+ Happiness since the Medieval/Renaissance age. Completely wiped out the 3 early aggressor against me. I razed all AI cities and built my own. I have double the score and no.1 in all demographics, except approval *lol*. I prefer to not conquer unless someone declares War on me or they are Hostile.

City-states
Definitely a good expansion for them. Some important fixes such as returning to neutral Influence after making peace. Giving Protection for 10 base Influence is good. I like how you have to take diplo hits with Civs in order to protect your Influence, and gifts, from City-states.

I guess it goes without saying that more Quests would be good but this is a big improvement on a City-state system that was rather bland and essentially an economic victory. It probably still leans towards that but it is much better.

COMBAT
Got dog-piled by the Iroqouis, Persians and Aztecs early on. Barely survived by managing to bribe Attila to fight them as well. Some what harder to fight the AI now with more HP per unit and that almost caught me off guard. Ultimately you just need to be more circumspect. Some people have complained that it is more of a grind, which it is, but an acceptable sacrifice to give the AI a leg up while maintaining 1UPT.

I have no idea whether it is viable to take a city without catapults as by the time I had seen off the initial dog-pile I had two catapults. A lot of gnashing of AI teeth followed I am happy to say.

ESPIONAGE
Not bad but not great either I thought. The global way they do it now is better. We don't need more unit maintenance as in Civ IV. I am also glad sabotage, water poisoning etc has been removed. Those things were just annoying little griefer acts that added little to the game play for too much micro management.

I like the potential idea for tech stealing BUT technology stealing feels like a cheap leg-up for the AI. Maybe if you are behind it keeps you in the game? Too soon for me to know whether I like it or not. It probably does make the game slightly harder if you are leading, but that is not so bad.

I have two equally vulnerable science cities and only have 2 spies so they are both stationed at home to try and mitigate further steals. Not much opportunity to manipulate any Intrigue as a result.

-------

Will probably change some of my opinions as I play more.