The cast of “Gossip Girl” caused chaos the other day as they filmed in and around the Empire Hotel on West 63rd Street. Police barricades and platoons of security men were needed to keep crazed fans at bay. One source at the shoot told us, “Taylor Momsen is way skinny. She is so much thinner than she was last year . . . her legs are sticks.” Off-screen couple Blake Lively and Penn Badgley were also at the shoot. A spy spotted them “making out in the lobby while the cameras were off,” causing “all the people waiting to get to the rooftop to gawk at them.” Tons of extras for the scene were spilling into the streets, donning flapper dresses and fedoras for the episode — which an insider told us “is about Ed Westwick‘s character throwing a Roaring ’20s party.”