Go to the Arasaka Waterfront in Watson, during the main story mission “Chippin’ In”
Follow Rogue’s lead until you reach the Ebunike
Defeat Grayson
Choose the dialogue option to spare him
Ask Grayson about the gun
Take the keycard he drops
Use the keycard to open the container nearby
Collect Malorian Arms 3516, Johnny Silverhand’s gun