In addition to what others have said the big problem with gas weapons is that once a gas has been deployed in an area there is no way to control where the wind blows it or how quickly it disperses. When ordering a drone strike the commanding officer has a very clear idea of exactly where the strike will hit and what area will be affected, because drone strikes have been tested thousands of times against every type of dummy target imaginable. This wealth of information about the behavior of a drone strike allows them to make clear and rational decisions about whether or not civilians will be affected. When you deploy a gas you have no idea where it will go, who it will affect, or for how long. There is therefore no way to make a rational determination about possible civilian affects.

Also, in regards to large scale war-time deployment of gas and chemical weapons, we have little to no information about what the collateral damage might be because things like spraying 5 million acres of inhabited jungle with highly concentrated and experimental herbicides simply had no scientific precedent. There was and still is no way to know what the long term effects on things like soil quality, plant and animal health, water supply, and of course civilian life might be.