After intensively calculating and trying to figure out the best cost ratio for gases I've finally concluded some stuffs.

Assuming fully researched them (mustard+working women)

6 gas (21 Pus) can kill 5 infantry (15 Pus) but gas performance starts rapidly declining if gas and inf stackt starts equally growing in terms of cost. Even in this case they are not cost effective at melting infantry stacks.

6 gas (21 Pus) can kill max 5 infantry (15 Pus)= 6 Pus defict

12 gas (42 Pus) can kill max 10 inf (30 Pus)=12 Pus defict

18 gas (63 Pus) can kill max 15 inf (45 Pus) =18 Pus defict

Even with techs their ineffectiveness is growing and other techs (propaganda+victory bonds+industry) starts to heavily outweight mustard+working women techs. Lets look at them with 3 Pus cost instead of 5.

6 gas (18 Pus) can kill max 5 inf (15 Pus)=3 Pus defict

12 Gas (36 Pus) can kill max 10 inf (30 Pus)=6 defict

18 gas (54 Pus) can kill max 15 inf (45 Pus)=9 pus defict

Still not cost effective. Yes they are mobile but you can already start benefiting them after mostly waiting extra 1 more round than as centrals and its enough time for entente to reinforce their key areas.

What if gas cost was 3.75 and infantry one 5? Also removing mustard tech. Lets take a look.

4 gas (15 Pus) has %70 chance to kill 3 inf (15 Pus). At this point they are effective wepon at melting stackt. and not enormously better than infantry considering gas is suicidal and can attack only one time also 3 infanty alsorequires less production capacity than 4 gas

8 gas (30 Pus) has just %35 chance to kill 6 inf (30 Pus). Well they started performing worse which I like it because it proves that even decreasing their relative cost %50 still does not make them broken or something.

As I said after calculating stuffs too much gas cost should be %33 cheaper than infantry. It is the most optimistic cost (alsono additional tech for gases).