Chernobylite is a horror game which relasede on 16 Oct, 2019 by The Farm 51. here is the guide on how to get Gas Mask.
Later on in the same mission he sends you to a building smoked out with green gas. Keep this is mind for later as for when you go exploring in Moscow that building also has the gas mask on the bottom floor. You can only access this building Via the Escape ladders.
Written by Mad Sweeney