Zwave Command classes are not designed that way. The command class itself is much more general, in this case it would be “alarm” or the newer “notification.” Then there is a parameter for that command which says what type of alarm/notification it is.
For example, here’s the conformance statement for the first alert CO2/smoke sensor.
Notice that it just uses the one “alarm” command for both its smoke and its CO2, but it does use different parameters for each, which is why SmartThings can process them separately.
And here’s a screenshot of the SmartThings developer documentation which shows the parameters available for notification.
It is then up to each manufacturer how they want to have their device report. So you would need to check the user manual or check with the manufacturer to find out what parameters they are sending for a gas alert.
Then, as you noted originally, you need to have a device type handler that knows what to do with that particular parameter.