Contract: The Oxenfurt Drunk - Novigrad
Quest Giver: Oxenfurt Notice Board
Other Requirements: N/A
You'll receive this quest by reading the Notice Board in Oxenfurt, after which you'll have to report to Nikolas Friedman (pictured above) and ask him about the contract. He'll fill you in on the details and give you two different quest markers to investigate.
One of the quest markers will take you to the bar to speak with the witness there and the second quest marker will send you to speak to the medic. After speaking to the witness at the bar you'll want to search the opposite side of the wall near the gate in the quest marker just outside the bar. Here you will find a bracelet which complete this step.
After speaking to the medic you'll have to examine the two corpses inside of the hut, once you've done that the next thing you have to do is head over to the bar and get drunk! Geralt will leave the bar after getting sloshed and roam around singing. Follow the quest marker on your compass until you come to an alleyway where you'll fight the Katakan.
Lowering the vampire's health to under 50% will cause it to knock your down and flee. It goes to an area north of Oxenfurt in a small hut, shown on the map below. You'll have to use your Witcher Senses to track it here.
Climb up the ladder on the northern side of the house and into the attic. Drop down to fight Gael, and this time it will be a fight to the death! He's still injured from our last fight so it shouldn't be too difficult. Once you've taken him out return to the contract issuer Nikolas Friedman to turn in the quest and receive your reward.
Katakan/Vampire Weaknesses: Vampire Oil, Igni