That's one type of approach, but the non-replaceable one would presumably eventually die.Such as 2 batteries : 1 built-in to run the system and one replaceable to charge the built-in unit. Or was that too simple ?
If it were a 'smart' meter (working in conjunction with a 'smart' electricity meter), the electricity meter could presumably 'report' if/when the signal from the gas meter disappeared.
Kind Regards, John