The expansion adds 10 cards to the game. Here is a description of the expansion from the publisher:

Through an exclusive arrangement with Geek & Sundry, Atlas Games released an expansion pack of 10 transparent cards for the Gloom card game as a free promotion for International TableTop Day 2013. Nearly 11,000 of these sets were printed. Its cardlist includes a cover card (the rules), 3 Modifiers, 1 Untimely Death, 2 Stories (The Losers Couch & TableTop Trophy of Awesome), 1 Event, and 2 Unwelcome Guest Characters (Wil Wheaton & Felicia Day!).