Chlorine gas is, of course, Cl2, not Cl-. I suppose that in principle if you had enough of the ion sitting around with the right other substances, you could manage to get it to react to form a poisonous gas, but then again, people don't seem to have too much problem with dying by standing next to brackish water (lots of chlorine from the dissolved salt) or hydrochloric acid, so I'd think the risks from that are slight. The hydrazine explanation sounds more reasonable to me, but it's been a few years since I was a chemist.
Actually, checking my various texts and the CRC, it looks like ClO- (hypochlorite) is a typical bleaching agent but is not overly involved with redox reactions in that the standard reduction potential is not large. As Squink says, ClO3- (chlorate) is another story entirely, but I would have thought that people who make bleach would want to create a product that isn't likely to lead to the formation of this less than entirely charming species.