You can obtain this in the level The VOLGA.
You must have a 4x scope.
You can find the night vision goggles in the terminal section (right side of the map - look for a ‘?’) - If there is no ‘?’ marker on your map then you need to progress the story further.
You will have the gas mask by this time.
Kill three enemies (humans or mutants) with the night vision activated & turned on, the gas mask on and while aiming down the sights of your weapon with the 4x scope.