Josh Ellis is a minor character in the third season. He is the Head of Freshman Affairs at NYU.
Josh Ellis is not a character in the Gossip Girl novels.
Josh Ellis makes his only appearance in Enough About Eve, as the decider of who will give the Freshman Toast at Parent's Dinner. His three candidates include Olivia Burke (first choice), Vanessa Abrams (second choice), and Blair Waldorf (third choice). After Olivia declines the invitation, he asks Vanessa, who readily accepts. Blair, wanting to give the toast, makes a deal with Josh to kiss Chuck if she can have it. He takes the deal, and kisses Chuck at a bar. Blair "catches" him, and he gives it to her.