You cannot install a gas fire in a room without dedicated non-closable ventilation. This is sized according to output of the gas fire, however, if there are other air consuming appliances, such as extractor fans, tumble driers, cookers, etc. in the same or adjoining rooms, these must be given consideration under the worst case scenario, i.e. all on at highest setting.
Balanced flue fires eliminate this need but some manufacturers may require additional.
You can also purchase a baffled air vent that will allow air to be induced into the room for combustion but will deter wind howling through. The most important factor for the vent is correctly sized & non-closable.
Another consideration is the flue. The integrity of the flue must be tested, with pellets.
Gas pipe & isolation is the other factor.
All of the above work must be carried out by an RGI.