Gas masks do give eye encumberance (strange as that sounds out of context) in the game afaik, gas masks get in the way of a proper cheek weld and eye relief looking down sights. Like with the M16/M4 you have to cant it to the left with the rear sight post bumping against the lens, it’s awkward as hell.
Then again, that probably shouldn’t matter that much, since all the gunplay in cata is panicky short range shooting on two feet anyways, all that shooting ergonomics stuff only matters for hitting things past 100 meters or so.