On TV or in movies a gas or spray containing a vaccine/cure/antitoxin is released and everybody is saved. Is this something plausible in real life? Specific examples would be appreciated.
Yes. This has been possible since the 1990's. In the US recently, the flu vaccine was recently offered as a nasal spray.
There is current research being done to aerosolize several vaccines. Public health officials have an interest in this method due to it being less expensive to produce, more people can receive the vaccine in a shorter time frame, and less need for infrastructure.
In India, they have started to give children aerosolized versions of the Measles vaccine. From research published in 2015 in The New England Journal of Medicine, scientists were reviewing how effective the aerosolized version is against the vaccine given subcutaneously. Even though the group's statistically analysis of the children's seropositivity, a method derived from previous studies, demonstrated the aerosol vaccine was statistically "inferior" to vaccines given sub-q. However, both groups of children had the same protection from the Measles.
There are a number of currently used aerosolized vaccines throughout the world. Generally, these are studied and administered in single individual doses. There are advocates for the use of larger, sealed exposure chambers for rapid administration of vaccines to large numbers of people, possibly in the field, for example in a tent like this figure from the earlier linked reference.
The primary barriers for a general release of a vaccine or other medication as, e.g., an aerosolized spray, would be ethical (difficult to get consent from all potential exposed individuals) and dose related (difficult to ensure a specific safe and effective dose).
The closest natural experiment I'm aware of would be the use of live attenuated Sabin oral poliovirus vaccine. Unlike other currently used vaccines, the Sabin vaccine is transmissible. In developing countries, vaccine is transmitted from vaccinated individuals to un-vaccinated individuals through the fecal contamination of food and water, using the same route that wild type poliovirus is transmitted. This can result in extending immunity beyond those who directly receive the vaccine. You could conceptualize this as blanketing a local population with vaccine. It's not without problems, though, because in cases where the overall initial vaccine coverage is low, circulation of the vaccine can eventually result in reversion of the reservoir of previously non-virulent (attenuated) vaccine to a virulent type. This is how you get vaccine derived polio outbreaks, and one of the reason not to use transmissible viruses as vaccines.