Does the fungicide gas drift around, or have an enormous radius? Does it revert fungal tiles to dirt?
I can definitely see it being useful for when you don’t want to outright destroy a fungus infested city, but everywhere else it seems like a couple of matches or a flamethrower will solve the problem more effectively, as fire is self-perpetuating and will rapidly consume the entire infested area, sometimes along with others that aren’t connected if its in a forest.
Unless the smoke drifts or spreads, and kills really fast. In that case it would be amazing and your mod would be my favorite ever. I hate having to go on regular fungus sterilization expeditions every time I spot one of the damn things, just to keep them in check.