After huge problems trying to buy XCom from GMG (and just getting the canned email response blaming my bank and PC settings), I've just bought the Civ 5 expansion without a single issue.
They gave me that same shit too on that Titan Quest Immortal Throne DLC. I payed via paypal. After a bit of back and forth, they finally said that they were out of keys and I'm put on the pending list until they get new keys this January.
BTW, Gods and Kings went through fine.