The Pilgrim's Peak Security Code is a temporary code used to unlock a main gate on the Blue Gate map. It is essential for opening a specific gate during a 'Locked Gate' map condition, as you need four different codes to fully unlock it.
You can find this code inside any lootable container within the Pilgrim's Peak location on the Blue Gate map. At least one code typically appears during a round, and you might find more. To use the code, proceed to the Gate Control Room on the Blue Gate map, enter the building, and submit it at one of the four terminals. Submitting a code consumes it and activates the terminal which helps to open the Locked Gate.