Instructions
Take the lift up to the city after escorting Krew's men through the sewers.
Make your way to the gun course northwest from the lift. A blinking icon on the mini-map indicates the exact location of the gun.
Hit the glowing Krimzon Guard box near the entrance to the gun course to reveal the Peace Maker gun attachment.
Walk over the glowing item to grab it. Jak will automatically attach it to his gun. Press the right button on the directional pad to make the Peace Maker the active weapon.