The only way Geralt can obtain this quest is by visiting Gramps' hut. Gramps can be there when he does, but Geralt must audibly acknowledge the human remains before talking to Gramps to receive the quest. As he does, he will notice half-eaten human remains, evidently "dinner" leftovers and remark that a cannibal must live there.
If Geralt discovers this before speaking to Gramps about his pilgrimage to the Shrine of Melitele, that quest is no longer an option during that conversation. To do both quests, Geralt should accompany Gramps to the shrine before going to his hut as the trigger is when Geralt looks around and sees gnawed bones.
Both quests are optional, but they also provide some useful opportunities for experience and loot.
A cannibal lives in a hut on the island. I must find him. I have to find the cannibal that lives in a hut in the swamps.
Cannibal Gramps Edit
|It turns out that Gramps, whom I took to the Melitele chapel, is a cannibal... I wonder what I should do with Gramps the cannibal... (500 XP) or|
|It turns out that Gramps is a cannibal. I found chewed human bones in his house. It turns out that Gramps is a cannibal. (500 XP)|
A Life Spared / Monster Edit
|Spare Gramps: The old man is a cannibal but I decided to let him live. Seems Gramps is a cannibal., or|
|Chat first, then kill Gramps: The old man is a cannibal. I will make sure he doesn't eat anyone else. I decided to kill Gramps the cannibal.|
Information / Death of a Cannibal Edit
|Spare Gramps: Gramps shared his knowledge with me, proving again that he's useful. Gramps shared his knowledge with me. (Azoth + Formula for Hanged Man's Venom + 1000 XP), or|
|Kill Gramps: Another beast is dead. Another beast is dead. (2000 XP)|