Your post does not explain your meaning of gassing.
"Vaccine" has a number of meanings. As a matter of fact, it has as many different meanings as the number of different things that can be put into the vaccine, multiplied by the age and condition of the person being injected with it.
Same with gassing. Gassing means making available for inhalation a substance or substances which have been gassified. The actual mechanism is identical in each case, while the number of substances which can be gassified, multiplied by the age and conditions of the people who are made to breathe said gasses, is exceedingly numerous and varied - just like "vaccines."
What's not to understand?