Using wall wart as 9v Battery Backup
Not knowing the circuit sensitivity to overvoltage and knowing wallwarts will be unregulated at 50% higher voltage with no load, 40% from rectified rms and 10% for DCR loss in transformer at full load, you can certainly choose your own appropriate supply if you understand the risk of overvoltage and need for reasonable regulation 10%.

Why do you have 4 gas meters? You mean Carbon Monoxide smoke alarms.  How many hours of power loss per 2 month? You could run a 9 V LDO at each meter from a 12V SLA and feed from spare wires in telephone jack if only using 1 line.  The monitors may need low ripple so measure both Dc and AC V to ensure proper performance with self test.

Another battery solution is 9 V Lithium which last 2-3 years or more from online sources. $10 approx. or use 3x CR123A Lithium in series @$1 each in bulk online.


added


From rough analysis in comments I calc. 1.6mA drain on battery backup for 8 mos life being powered from internal AC-DC supply due to automated battery test, (guess)  Lithium 9V , 3x to 5x capacity of Duracell depending on specs . avail at Lowe's $11. **I might consider 1k to 330R to internal 9V dc supply but measure to null current drain from battery over a period.*