On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the Choking the Gallows mission.
Eivor has obtained the necessary information to expose the first of the targets from the Order. Time to go to the marketplace and eliminate them.
Reward: 1,700 XP.
Assassinate the Gallows
With Goodwin's help, another member of the Order of the Ancients was exposed. It is time to go to the marketplace where a public execution is about to happen and kill Selwyn, who is hiding behind the pseudonym "The Gallows".
After the execution, The Gallows will stay on a wooden platform so you can go in from behind and eliminate him with the Hidden Blade. If you attack from the front, he will defend himself or start running away, and there will be extra guards on your tail.
After the murder, the mission will end and you will receive 1 700 XP as a reward.