Fields of Despair - GMT P500.
1. Does either player have to announce when they have developed Chlorine Gas tech?
2. Can a faction that has developed Chlorine/Mustard have to use it during an Artillery attack or can they hold it back in order prevent the other player from starting research on Gas Masks, assuming the other player has not developed Chlorine/Mustard gas tech themselves?
No announcement, but the first time you roll a "4" with artillery you must tell them that a gas hit has occurred. There's no rule forcing you to announce when you have advanced to the tech but you cannot disregard the gas hit as it is automatic once the tech level is reached.