Wrong. If that's true why did the British government issue them to civilians during WWII? A gas attack even by aerial bombing doesn't deliver sufficient concentrations of the agent to overcome the average person within 4 seconds, (the maximum time that should be necessary to put on a gas mask). On the tube the gas dispersal system is likely to be very crude, probably a reaction mixture in a container, which would be thrown into a train carriage or onto a platform. Also, cyanide producing chemical reactions often produce a whitish vapour and an almond like smell, hence you would know something was going on before a lethal concentration of the gas got to you and so you could put on the mask before you were overwhelmed.
Of course, a biological attack is a completely different matter. You would have to wear your gas mask all the time while you were on the tube if you wanted to be sure you were safe from a microbial agent, since you would have no warning that the agent had been released. Strictly speaking only a military NBC-grade gas mask is guaranteed to give protection against biological attacks.
Anthrax is probably the most likely biological agent to be used, less likely are tularemia and plague, these two are more difficult to mass-produce and disperse as they are gram negative non-spore bearing coccobacilli. However, these two bacteria would cause a dangerous epidemic, since unlike anthrax they would spread to other people who were not infected in the attack. Mass production and freeze-drying of anthrax spores, under safe conditions, isn't that easy and I suspect al Qaeda would need help from Iraq, or some other scumbag nation, to mount an anthrax attack. A gas attack would be a lot easier and wouldn't require stuff to be imported, if the perps know anything about basic chemistry.
A Smallpox attack on the tube would be the worst case scenario, because by the time the first cases have been diagnosed and the alarm raised - hundreds of people all around Britain, and probably across Continental Europe, would already be infected. The Soviet's India-1967 bioweapon strain of Smallpox is the most lethal strain available, (at least a 50% mortality rate in unvaccinated victims). Unfortunately, this is the strain Sodem Insane is most likely to have, since the Russians kindly supplied it to North Korea and probably their Iraqi pals too. The British Government should buy in enough Smallpox vaccine to do the entire population and should make the vaccine available to any Briton who wants it. That would make mass vaccination easier if a Smallpox attack occurs, as well as not tying the fates of all Britons to the decisions of a bunch of incompetent PC left-wing half-wits.