You will need to find all the keys to unlock the Slayer Gates if you plan on getting the platinum trophy for Doom Eternal. You will receive the Bonus Stage trophy for just completing a Slayer Gate and will receive the Breaker of Gates trophy if you complete all Slayer Gates in a single save slot.
Mission 2: Exultia
Location: Hell (Travel Point – Precipice of Recompense)
The key for the Slayer Gate is located on a platform floating just above the gate. To reach it, enter the cave to the left of the Slayer Gate and work your way up to the top ledge to find a hole above the entrance of the cave. Head through the hole to reach the platform on the other side.
Mission 3: Cultist Base
Location: Cultist Base (Travel Point – Promenade of Culling)
In the room with the demon effigy, look down to the left of the effigy to find a climbable wall. Jump down onto the climbable wall and look behind you to find two gold spin bars. Use them to leap to another wall directly ahead. Turn to your right to see a yellow recharge floating in the air. Dash to the climbable wall past it and climb up it. Turn around and jump into the room then use the yellow spin bar to jump up to the small room containing the key.
Mission 5: Super Gore Nest
Location: Upper Area – Mixom Square
From the fast travel point, proceed forward, passing a gunk covered column, to reach a blue air stream in the middle of a gunk tower. Use the lift pad and turn right to find a long climbable wall. Avoiding the debris, jump to the climbable wall and work your way up it to reach the room at the top. Work your way to the end of the room to obtain the key.
Mission 6: Arc Complex
Location: Vargo Convention Center
From the starting point, turn left and head towards the brown door to find a set of stairs. Take the stairs up to find the Slayer Gate. Head to the edge of the broken walkway and jump to the climbable wall on the right side. Turn around and face the opposite direction then jump to the room containing the key.
Mission 7: Mars Core
Location: Meteor – Delta Labs
From the fast travel point, head left in the facility through a doorway. Once through the doorway, take a right and interact with the console to raise a pillar from the ground in the previous room. Head back through the doorway and take a left to see the pillar. Use it to break the vent shaft on the right side and enter it to find the Slayer Key.
Mission 9: Taras Nabad
Location: Catacombs
From the fast travel point, work your way through the sewer section of the catacombs. Before exiting the tunnels of the catacombs, take a left and head up a ramp to find the Slayer Key at the end.