Gregor was a ferocious bird that kidnapped Julien (to eat), and framed Kitka for the crime.
He swooped down in the night, knocked Mort to the ground, and snatched away Julien. The next day, the Penguins (along with Mort and Maurice) ascend the Consolidated Amalgamated Building in Midtown to save him. They find Julien cringing on the ground, and the bird attacks. As Gregor zooms in on them, Kitka clashes with it and spars fiercely, but is no match for Gregor. Kitka is knocked to the ground, and Gregor turns on the penguins. The penguins try and even Maurice joins in, using Mort as a nunchuck. But Gregor prevails, being able to dodge and strike at the penguins from the air.
After Skipper realizes that Gregor is more than they can handle since the bird is in his element, he follows Gregor through the air on a grappling hook that is attached to the falcon's leg. Julien meanwhile is describing the battle to Kitka by mimicking "really cool sound effects". Back in the skies, Skipper leaps onto the Gregor's back and kicks him repeatedly, yelling "YOU WILL NEVER FOUL THE GOOD NAME OF MISS KITKA AGAIN!". Gregor however, is quick to hurl Skipper to the ground. Skipper manages to hit a flagpole and rocket back up to stop Gregor. He grabs his neck and directs him over the building and into a heating funnel. Skipper manages to leap off Gregor before he crashes through the pipe, and falls to an unknown fate. It is possible that he survived.
Kitka: (gasp) I can't watch! Julien: Okay, I will describe it for you in really cool sound effects, check it out SCREAM! SKRAAAWK! OOF! Gregor: SKNEEEEERRRE!!! Julien: flip-flap-flippity-flip-flap, OOOHH! I AM A PENGUIN falling to my doom! Kitka:(gasp!) Julien: No, no, no. I was just practicing that one in case I need it later, but I got you though didn't I.