Gods & Kings adds two new mechanics, religion and espionage, and rebalances the tech tree, as well as adding 9 new civilizations. There are also several new scenario packs, but generally people don't care about those. (The steampunk scenario is pretty cool...for the one hour that it takes you to clear it.)
All of the new updated gameplay changes included in Gods & Kings are also included in the later expansion, Brave New World. Basically the only reason to get Gods & Kings at this point is if you want access to the unique civs that are included in this expansion. So I guess it's worth it to buy Gods & Kings when it's on sale if you want a larger pool of civs to choose from, but if you're going to pay full price for a Civ 5 expansion pack just skip this and get Brave New World instead.