Gertarh's corpse can still be found in his den in the locked portion of the dungeon. The key to access his den is found elsewhere in the dungeon but strangely it only unlocks doors beyond the tattered sign - you will need lockpick for the main door.
The bottom most part of the den, known simply as the pit, was once a popular levelling spot for low levels and friends to hunt at.
The Alloy Cogwheel that is used in the Undead Mechanic quest can be found in the pit at the very bottom of the dungeon (See Map). Numerous loot spawns, chests and even a corpse containing loot can be found in the den.
Chest (Tier 4) - multiple chests found in the pit area
Key (Thieves' Den) - found on the floor in the south west lever room. Unlocks some of the doors to access Gertarh's den and also one of the cells in the den itself.
Quest Items:
Alloy Cogwheel - found in the north room of the main pit