The song in the promo video for Gossip Girl Season 5 premiere is called
"Kinda Outta Luck" by Lana Del Rey (see first link below)
The song we hear when we first see Dan in the premier of Season 5 is called
"Shuffle a Dream" by Little Dragon (see second link below)
If neither of these songs are the right one, go to the third link below for a list of songs for each episode of Gossip Girl this season.