Talk to Gamma in monitor 3.
Disable up to three of the monitors with the PLASMA TORCH to try and trap Gamma in the last monitor. However, he keeps moving around so you could kill him if you accidentally destroy the monitor in which he is hiding. Thus, there are two ways in which this can play out:
1) If you destroy the three monitors that Gamma is not hiding in, you will trap him. You can then go and tell Alpha and Beta that Gamma is trapped. When you leave, Goliath will wake up and give you a DECRYPTION MODULE. You will also get the Defragmented achievement. To make sure this happens you can best come back later once you have the mobile sensor. (see below under “The Dome”). Use the energy sensor on the monitors to find one that says it’s not detecting any energy. Then quickly destroy that monitor, and so on until there is only one left.
2) If you accidentally destroy the monitor in which Gamma is hiding, you shut down the whole system and kill Goliath. You will get the Call Me David achievement, but you won’t get the decryption module. You will be able to complete the game without it, but you’ll only be able to play the “alternative” endings and miss out on information regarding Horatio’s past.
In both scenarios you will find out that Horatio’s real name is Horus when you leave the scene, which is important for the next part of the game.