GPS Secrets refer to special collectible Documents found in Tomb Raider.
There are only two of these Documents found in the game.
Unknown: Mysterious Orders[]
Command to Operative
STOP
Trinity is concerned.
STOP
Acquisition of Star Phenomenon unacceptable.
STOP
Sabotage of Axis operation imperative.
STOP
Lara's Thoughts[]
Was there some other person... or group... interested in this island? I don't have time to make sense of this now.
Unknown: Failed Mission[]
I was sent here to investigate a new threat to Trinity's interests on this island. When I arrived, I discovered Mathias and his Solarii Cult. I believed then that Mathias could never acquire the Star Phenomenon, but I was wrong. I should have killed him when I had the chance, but now it's too late.
I've been compromised and mortally wounded, my mission incomplete. My time is short.
If you've deciphered the numbers and found this stash, then you must be my replacement. Kill Mathias! He's close to reawakening the enemy that has long lain dormant. This cannot be allowed to happen.
Lara's Thoughts[]
That name again... Trinity. Some kind of organization. But who are they? And how do they know about this island?