Central Power's biggest production locations (Berlin, Mecklenburg, Vienna,Hungary,Constantinople) are already too far making noticable effect in battles. Entente nations start immiadetaly benefiting inf/conscript spawns because they provides immiadetaly cheap defensive power while gases from these locations don't provide immiadeate benefit.
For example bought 26 gases in round 2 from Berlin-Mecklenburg will be ready attacking Paris in round 4 while paris infantries at the same time provide immiadetaly benefit. In round 4 these gases will only be able to kill roughly 17 inf (it means burning 104 Pus for just taking 51 Entente Pus)
Wile France in round 2 and round 3 will have extra minimum 19 units+6 trences even these purchases are enough to negate all off these gases let alone considering other French units, possible defensive techs and UK+Italian supports.
It it true they can threat multiple targets but also buying more gas also mean less defense, less firepower, less tech etc. Enemy may decide retreaing and hitting back or sacrificing some stuffs. It is still very bad unit.
Without any tech gas is incredibly bad unit. 3 gas (12 Pus) takes on just 2 inf (6 Pus) two times more ineffectiveness!.
The only reason buying some of them is rushing some important locations or minor nations in initial rounds after that there is zero reason buying them if nation is not going to take on a major enemy capital.
Or as France why would I buy gas?