There’s a thread on the civ 5 forums about the expansion, various things people have dug up: http://forums.2kgames.com/showthread.php?118092-Gods-amp-Kings-Super-Sleuthing
some highlights:
Release date “late spring”, estimated cost $29.99
9 new civs
9 new wonders
27 new units, 13 of which are UU for the various new civs and some of which (maybe the rest of) are probably for the new “steampunk” scenario.
13 new buildings
A new era between renaissance and industrial
New city-state types, religious (gives +faith) and mercantile (gives unique resources)
Unit HP changed to 100 (from 10), apparently to make combat last longer and allow more room for strategy. Not really sure if unit damage is going to be changed or if it’s going to take 10 times as long to kill everything or what.
Embarked units can all defend themselves now, dunno what they are going to do with songhai (This change is probably because the AI is completely incapable of defending it’s embarked units, which kind of makes me sad)
Spies are not units on the board, you get 1 per era (starting in renaissance) and you attach them to enemy cities (including city states) to do various things like steal tech and rig elections. They can gain levels like other units, and if killed you have to wait a while before they are replaced.
Religions work sort of like social policies, you start out with a sort of basic polytheism and generate “faith” which lets you upgrade your religion to one of 11 possible choices, apparently each one has unique buildings and such but there’s not much known about it yet.
3 new scenarios: an unnamed medieval one, fall of rome, and a scifi steampunk styled one.
Supposedly upgraded diplomacy and AI… which I’ll agree is BY FAR the most important part because the AI right now can’t really seem to cope with 1 unit per tile at all rendering the biggest improvement to the game moot, and currently diplomacy is basically non-existent (every AI will eventually backstab you at some point so there’s no reason to ever co-operate with them past selling them your extra luxuries).
There’s lots more information on the linked thread so you should check it out, it certainly seems interesting and I’m starting to really look forward to it.