The Gallery is divided into two parts: The existing gallery and the ancient smuggling tunnels below. The Gallery has three floors containing minor loot and plenty of Slab's raiders. The tunnels themselves are fairly straightforward, if winding, and ultimately lead to a confrontation between Pickman and Slab. If Pickman survives the shootout (unlikely without the player's intervention), he will give the player Pickman's key and start Pickman's Gift.
↑Fallout 4 Vault Dweller's Survival Guide Collector's Edition p.465-456: "[13.01] PICKMAN GALLERY A prolific serial killer stalks the alleys of North End and has recently been cornered in his lair by his prey of choice: Raiders. The rotting hulk of a row house Pickman calls home was chosen for its access to ancient subterranean smuggler tunnels beneath North End. He displays pictures of his victims on the walls. Inside the dwelling, his blood-splattered daubing and corpse sculptures show the type of lunatic Pickman is. He has left a message to Jack in the front room and Pickman’s Calling Card on many corpses. Fight your way to the end of the smuggler’s sewer." (Fallout 4 Vault Dweller's Survival Guide Map)