Pickman's Gift

Pickman's Gift is a side quest in Fallout 4.

Detailed walkthrough
Upon entering the Pickman Gallery, the player character will come across multiple raiders, searching for Pickman. They are hostile to the Sole Survivor. In the room immediately to the left of the entrance one will find the body of a raider that has the holotape Message to Jack on it. Listening to this holotape will give the first clue about Pickman.

There are two different ways that one can enter the hidden tunnels beneath the gallery. One is to find the hole in the wall on the third floor and use it to drop down into Pickman's basement, which then leads to the tunnels where one can hear someone calling for Pickman to come out of hiding. Alternatively, one can try to pick the Master lock door which also leads directly to the same place.

Continue through the winding tunnels while fighting off numerous raiders and a turret. When arriving at a ledge at the end of the tunnel, the Sole Survivor will find Pickman being threatened at gunpoint by several raiders, including their leader, Slab. The Sole Survivor can either save Pickman, let the raiders kill him, or kill everyone. Even if they let the raiders kill Pickman, the Sole Survivor will still have to fight them afterward to progress.

After killing all the raiders in Pickman Gallery, one should either speak to Pickman or loot his body (if he was killed). This will grant the player character Pickman's key, upon which the quest will be added to the Pip-Boy.

If Pickman is spared, he will express his gratitude and tell the player character to take a closer look at his painting "Picnic For Stanley" for a gift. After exiting the house via the tunnel exit nearby, re-enter the gallery through the front door. In the northwest corner of the ground floor is a painting of a man with his eyes removed — "Picnic for Stanley". Activate it to reveal a safe that can be opened with Pickman's key. Collect the gift from the safe to complete the quest.

Bugs
If picking the lock on Pickman's safe behind the painting and taking Pickman's Blade before saving or killing Pickman and then the Sole Survivor encounters Pickman, he will still give the key and the player character will have the same conversation options triggering the quest. After deciding Pickman's fate, and returning to his safe, it will show nothing in it and still be unlocked from the way one left it earlier. The quest will still be in the log though. To fix this use the transfer option and place Pickman's Blade back in the safe, exit the safe, re-enter the safe and retrieve the blade again. This will mark the quest as completed in the log and award with experience.