Seismic gas shut off valves are intended to automatically shut off the gas to a home in the event of a major earthquake.
In some cities they are a requirement, others they may only be a recommendation, and in some areas of the country they are not needed. A call to your local building department will provide their requirements for your area.
There are various sizes of valves and some are designed to be installed vertically and some horizontally.
Who to consult? Plumbers, qualified handymen and seismic valve installation companies.
DIY home owners can do some installations, but the majority of time plumbers, qualified handymen and seismic valve installtion companies do this type of work.