Supposing that the tensions that led to WW1 still exist but there's no spark to set everything on fire, would it be possible for there to be an atmosphere similar to that of the Cold War, with poison gas taking the place of nuclear weapons?
That's difficult to achieve. It wasn't just the nukes that caused the Cold War standoff, it was also the intercontinental delivery systems (bombers and missiles) that allowed those nukes to threaten everyone. Without that, you don't have a global threat forcing states into semi-aligned unipolar blocs. You might be able to get a sort of mini-Cold War in Europe, but there would still be far more actors in play, and there would still be extra-regional powers more or less unaffected by whatever poisonous threat was hanging over the area.
I think that, as with nuclear weapons, you have to have them used in some public fashion for their terror to become palpable. Remember, after Guernica, the fear wasn't just that an enemy airforce could destroy your city, but that they might drop gas weapons - but this was only a real threat and fear because of their use in the battlefields of the First World War.