Depends on where the heater is located.
If it is inside your garage and subject to outside temps.......yes. If it is inside not subject to outside temps then probably not.
The real issue on water heaters is the top and top 1/3 of the tank. Since gas heaters have a vent out the top, it is pretty hard to put any insulation on the top with out running some risk of screwing up the air flow.
Also if outside then you might want to insulate the water lines as well.
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this stuff is not fire proof.
I used something like this on my last electric heater. Seemed to help, I only ran the heater 2 hours a day usually.
Be damned careful about the vent and all of its properties.