Not gonna happen. Gas can only be used by Presidential authority and even then only when it's been used on us by another nation first. Terrorists are not a nation and they do not stand and fight like a regular military. Thus the term "terrorist". So even if they used gas, toxins (i.e. anthrax, etc.) we won't.
"Do not forget your dogs of war, your big guns, which are the most-to-be respected arguments of the rights of kings."
- Frederick the Great, King of Prussia