Silent Assassin is a gear perk that raises the damage of silenced weapons. Suppressed guns lose some punch, so this helps you get one-shot kills while staying quiet.
Shots from a suppressed weapon hit harder. You need fewer rounds per kill, so guards have less time to raise the alarm.
Wear the Posh Gloves in the hands slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Stalker and Travel Light cut your movement noise, which completes a quiet build.