While collecting protection payments for Rais at the Fishing Village you talk to one of the villagers and learn that a local crazy called Gas Mask Man is cutting the locks off and opening their gates, which is how the infected ended up invading. They need you to deal with this wacko!
When you confront the guy with the Gas Mask - an Assault Rifle - he comes off as totally crazy - but he also claims that someone ELSE did the sabotage to frame him.
You don't know yet whether that is true so you end up doing a task for him by retrieving a package from the water - a treasure bag...
After you swim out and dive down to recover the Treasure Bag, you head back to Gas Mask Guy and have a chat, during which he gives you a series of observations that are rather disjointed. He tells you that the imposter is one of the fisherman. Then you learn that his treasure is... Air... This guy is nuts!
Still your next task is to Talk to the Guard - so get to it!
When you talk to the Guard you learn who has keys to open the gate - then you need to talk to them: Tom, Musa, and Gursel.
Not sure if Gersal will be the villain but we found the mask in his house and, when we confronted him he admitted it.
You convince him to make this situation right - Gas Mask Man may be nuts but he is harmless nuts right?
Wrap things up by talking to Musa.
Collectibles
Battle Journal Entry #07 - In a car trunk next to Gas Mask Man's house.
Note #09 - In Gersal's House