If faced with trying to kill a large number of mutants or alien abominations, like a horde of giant ants, bees, or a swarm of Mi-go, indirect warfare methods would be preferable to direct engagement.
For example, it would be way smarter and safer to just dump poison gas into an anthill than fight your way through it Starship Troopers style to assassinate the queen.
For gameplay I’m thinking this would add:
40mm Gas grenade variants for the different agents.
Gas hand grenades.
Gas canisters for storage of gaseous chemicals.
Gas barrel bombs for large area dispersal.
Gas mortar shells (if I or someone else ever gets around to making mortars).
Gas Sprayers, a remote controlled poison gas sprinkler for base defense.
While many modern nerve agents like Sarin or VX would not be something a survivor could manufacture, (though it would not be impossible to FIND some in a military facility, although they would be very rare), there are a number of chemical weapons that have been used in real life conflicts that are simple enough to manufacture and weaponize that you could do it in your garage.
Chlorine Gas would be the simplest example. You can make it by mixing bleach and hydrochloric acid:
HOCl + HCl
H2O + Cl2
or by mixing bleach and vinegar:
2HOCl + 2HAc
Cl2 + 2H2O + 2Ac- (Ac : CH3COO)
or a number of other acids.
Phosgene Gas is manufactured by reacting carbon monoxide and chlorine through a porous bed of activated carbon (aka charcoal), while the reaction is cooled. The carbon monoxide gas could be relatively simply prepared as well with basic lab equipment by reacting steam with carbon.
H2O + C → H2 + CO (ΔH = +131 kJ/mol)
To make the phosgene, you would simply need a sealed refrigerator, charcoal, the reagents, and some other misc. components like tubing, sealant, etc.
CO + Cl2 → COCl2 (ΔHrxn = −107.6 kJ/mol)
Mustard Gas may be manufactured via Ethylene and Sulfur Dichloride.
Ethylene is manufactured by the dehydration of ethanol (alcohol) with sulfuric acid.
Sulfur Dichloride is manufactured via the chlorination of sulfur.
S8 + 4 Cl2 → 4 S2Cl2; ΔH = −58.2 kJ/mol
They are then combined via the Levinstein process to yield Sulfur Mustard
8 S2Cl2 + 16 C2H4 → 8 (ClCH2CH2)2S + S8