SGSOV stands for Seismic Gas Shut Off Valve. The SGSOV will automatically shut off the gas to a home if there is a 5.2 or greater earthquake. This SGSOV is independent from the normal manual gas shut off valve (located here) that is on every home, usually located near the ground

This brand of SGSOV is called The Little Firefighter, and is common in Southern California, where many cities require an automatic earthquake shut off valve when selling. More on the different types of SGSOVs that are common in Southern California here.
There are various models of SGSOV valves, but the Little Firefighter type is the most common in Southern California.
From the Little Firefighter Website: This easy-to-install device helps prevent gas leaks and fires by automatically shutting off the gas supply to a property in the event of an earthquake or other seismic activity from a significant earthquake. The valve works by detecting significant shaking motion caused by seismic activity and immediately shutting off the gas flow to the structure and all connected appliances and equipment, including stoves, water heaters, and furnaces.
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