It's probably in the dispersion system. Judging by the look of the original dharma people that Ben killed, the gas is unleashed either very, very quickly or everywhere on the island at the same time.

I agree it was some type of failsafe installed by the original dharma people to neutralize everyone on the island in case something happened. Ben obviously knew about it. The freighter people knew that he knew about it and wanted to shut it down cause they thought Ben would gas the whole isand in order to protect it from outsiders.