In world war 1, about 90 000 deaths were caused by chemical weapons (i.e. gas). This is a small percentage of the total and only a small percentage of gas-related casulaties were fatal- only about 3% of gas-related casulaties were fatalities, and it's estimated that 70% were ready for action again within 6 weeks, mostly as the gases used at the time were not extremely effective and countermeasures such as gas masks were available.
In world war 2, gas was not used as a weapon to any significant extent outside of the murder of civilians during the holocaust. The Japanese used small amounts in China, and Germany accidentally dropped a gas bomb on Warsaw in 1942. Most major powers involved in the war stockpiled gas weapons but they were never used. There were, however, several deaths related to accidents relating to chemical weapons- e.g. when the Germans bombed the port of Bari, Italy, they hit an American ship containing chemical weapons which leaked out, causing more deaths.
It's much harder to estimate the number of lives saved by gas masks, because unlike deaths, it's much harder to actually count the number of lives saved, and it's also very hard to estimate who would have died if they had or had not had a mask available. Also, the fact that gas masks were available also influenced the decision by enemy forces to use gas weapons in the first place, and it is often difficult to calculate how effective these would have been.