Pictured is a "flat/square head" valve knob. Its purpose is to prevent operation by hand or by accident, but to allow operation by almost any large enough wrench or flathead screwdriver. You would use it where you want to prevent accidental or mischievous operation but without providing any real security or requiring special tools or keys.

Wilkins NR3repair kit

For limited access, a ring-spanner of appropriate size should get you in there eventually, but you might want a ratchetting one or patience. Your access looks good in the photos though.

Zurn WilkinsPressure Reducing Valve

3/4''water Pressureregulator

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And the final option is to cut a disk of wood, and drill-then-file a square hole to take the square shaft. This gives you a handle that could be left in place for quick action as long as the required hand-pressure is not excessive.

They sell specific "gas shut off tool"s for shutting off gas valves in emergencies. For example, this is a common design that can be used to shut off both gas and water:

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Wattspressure reducing Valve Repair Kit

I was wondering how to use this gas shut off valve. It looks like it requires a special type of key but I have had no luck in finding out what type of tool I need.

How big is the hole through the side? Leverage can be gained by using a screwdriver or even a suitably-sized nail through that hole and (presumably) the one opposite.

NR3XLpressureregulator

Zurn NR3XLRepair Kit

Personally if I was doing this a lot, I'd use parallel-jaw plumbers pliers like these knipex ones The only downside are Price, and that you have to reset every 90 degrees of turn. Minimum is 60 degrees and a lot of resets.

Looks like a plain old adjustable open end wrench (or an open end wrench of the correct size) will operate it. Wrench on the square bit of the red part, turn the whole red part, probably 90 degrees.