Is building regulations approval needed for replacing gas fire ... ?
Permanently removing a gas fire involves capping off or removing the gas pipework and testing it to ensure it is safe.
80 Gas work should only be undertaken:
(a) by a person who has successfully completed an industry-recognised
training course followed by assessment of competence. Training that
leads to assessment of competence in safe gas work should be
recognised by the industry’s standards setting body; or
(b) in the case of a currently or previously registered person, where they
have proved competence through a certification scheme; or
(c) for those working at premises that fall outside the scope of the
Regulations (see regulation 2(4) and associated guidance), by a person
who has successfully completed an appropriate full training course
followed by assessment of competence.
Gas Safety (Installation and Use) Regulations 1998 Approved Code of Practice and guidance
Unless you are a landlord, there is no legal requirement for a certificate for the gas installation.
My guess is that the buyer wants evidence from a qualified engineer that any remaining gas pipes and equipment are safe and that the chimney flue was properly adapted for the change in use.
Whether you pay someone qualified to inspect and report on the work depends on how much you want to appease this particular potential buyer. I guess it is something that is likely to come up again though.