Not scientific. But, if you look into history it’s arguable that the exodus of the Hebrews was started after a massive volcanic eruption.
Gas that kills “first born”? Probably more than that if it was methane release. Reptiles jumping out of the river, filed with “blood” or lava. A fissure that could cause the Red Sea to shift and leave an area to cross?
Of course, this would be over years. But doesn’t the Bible tell stories over years?
Just a conjecture for a thinking excercise.
It wasn’t “gas” that killed the firstborn, it was the Angel of Death. Besides, if it had been some low-concentration poisonous gas, it would have been more likely to kill the youngest children than the eldest.