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.
If you decide to kill Gramps, after meeting him in his hut, he will run outside. Once you go outside too Gramps will have backup in the form of a circle of 5 Echinops and a unique Archespore in the middle (has Gramps been growing them?), so be prepared for a tough fight. If you are low level then best idea is to stay hidden behind the hut and wait for Gramps to come to you, then kill him and next kill off the weeds one at a time using Igni and running back behind the hut. While there is no specific oil that works against Echinops or Archespore's Argentia will increase your damage against them. Also if you have not yet killed the Coccacidium then it will most certainly join in too, so it's best to kill it before starting this quest.
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.
- 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)
- 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
- Spare Gramps: The old man is a cannibal but I decided to let him live. Seems Gramps is a cannibal.
- 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
- 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)
- Kill Gramps: Another beast is dead. Another beast is dead. (2000 XP)
- If you are having problems deciding between the Formula for Hanged Man's Venom and doing what's morally right, remember that every recipe in the game can be obtained by experimenting. You can actually unlock the recipe as early as Chapter I with that method. In fact you can "rub salt into the wound" by using Hanged Man's Venom in your fight against Gramps.
- The Archespore that appears during the fight with Gramps is different from regular Archespores in that it looks just like a large Echinops and doesnt seem to drop anything even if you have entries for both types of monsters.
- Gramps' man-eating habits and odd relationship with the Echinops may be a reference to the Little Shop of Horrors.