I love this excuse. It is made by legislators all the time when they propose stupid legislation. Sometimes it is true, but mostly it's an inept excuse.
Illinois Senator Bill Brady used this excuse when he came under fire for proposing a law that would allow animal shelters to stuff as many animals as possible into a gas chamber. Currently, one animal at a time may be placed in a gas chamber and, according to McLean County Health Department spokesperson, Erin Tolle, the animal just falls asleep all happy and cozy as a bug in a rug!
But then the HSUS was all, um, for reals? and Senator Brady was all OMG A CONSTITUENT MADE ME DO IT, U R MEANZ!
I think gassing can be "humane" when it is used to euthanize irremediably unhealthy animals. Just like I think euthanasia solution can be "humane" when it is used to euthanize irremediably unhealthy animals. Can be, being the operative phrase.
There is nothing humane about killing healthy animals.
As far as welfare is concerned, there are pros and cons to each form of death. Gassing requires a good system. It works best when only one animal is in the chamber. A geriatric animal will react differently than a neonate than a juvenile. A stressed animal will react differently than a calm one. Placing multiple animals into a gas chamber ignores this fact and results in some animals being rendered unconscious, not dead. The machine has to be in good working order and supplying the right concentration of gas to induce unconsciousness, then death, and avoid anxiety and avoidance behaviors.
Euthanasia solution works best when the animal is calm or tranquilized. If the sedation occurs in the kennel, that eliminates the stress of moving the animal. If not, movement of animal can be as stressful as the actual injection. It requires more skill in finding a vein for adult animals, which also requires physical restraint, a stressful process (you can go IM, but it takes longer and isn't really rec'd, IP is rec'd for poultry and neonates). It is more expensive. It can be more or less gentle than gassing.
For an animal who wants to live, neither are very appealing.