I am not a gas engineer but I do employ gas engineers who do this work.
So
I wanted to know the answer to this question too and it wasn't easy to find a reliable source for the definitive answer.
It was a teacher who told me same as what others have said in this thread.
So if there is a fire/ cooker you need to be qualified to work on those.
I think there is genuine ignorance on this particular question.
Or perhaps convenient ignorance. Because most of the time there is only a boiler in the property Many, many times I have heard of Landlord inspections being done and completely ignoring the cooker or fire, the engineer or employer had not anticipated that those appliances might be there, the job would take longer (but they are only charging pennies) andd the gas engineer does not have the right skills.