There is indeed a NATO symbol for chemical weapons (or to be specific, nuclear, bacterial and chamical weapons). According to the
US Army Field Manual FM 101-5-1/MCRP 5-2A it's this:
As for whether to include gas artillery, unless anybody has a really good idea of what to replace it with I'd suggest leaving it as is, but making its abilities almost identical to conventional artillery, except that gas should have a higher soft attack and normal artillery a higher hard attack/anti-fortress ability.
Although very few people were killed by gas, it wasn't actually designed to kill people... if a large proportion of your opponents are blinded and choking, another section are forced to help them to the aid stations, and the rest of the troops are muffled up in hot, heavy respirators, they're not going to be resisting your attack very energetically... One key to the successful German and British offensives of 1918 was extensive use of gas shells as part of the hurricane bombardment just before the attack, to disorientate and unbalance the defenders...