Cat convertor fires require a min room size and min airflow.
The most efficient fire is a Balanced Flue - they are a tube through- the wall type. They draw in air from the outside and pushing out products of combustion, so about 90% efficient. Best is Rinnai (Japan) re computerised 3.3kW output, fan heater, electronic timed start. Cheapest I think is a Robinson Wiley. These do not use your chimney.
The next most efficient is an Outset Radiant Gas Fire. The Radiant Elements are heated by the gas flame and glow yellow- orange, flooding the room with radiant heat like the summer sun. The radiant heating means you can typically run a room at 18-20oC, some way below the temperature required by purely convective heating means. The Outset Design means the fire sits outside the chimney area, with
56-82% efficiency (I think there is one at 87% efficiency). They act like a big convection heater, limiting heat loss up the chimney.
The least efficient is an Inset Gas Fire. Typically these are 28-38% efficient and little better than a "gas burner at the bottom of a wide open chimney" in terms of sucking cold air through the house AND are very high kW input so do require a gas vent in the wall which will make the room cooler.
A 7kW INPUT gas fire does not require a dedicated gas vent in a wall (creates draughts) if, and only IF, there is sufficient airflow through the house. An example would be single glazing and wall vents elsewhere, plus tested with all shower/bathroom/toilet, kitchen & tumble dryer extractors running flat out.
An inset "decorative flame effect" gas fire may have to draw 22kW to match the room heating effect of a 7kW output outset radiant fire, and the draughts may mean it never does.
You need to contact a Gas Safe to 1) check the chimney is ok 2) check the gas supply pipe is suitable and that goes for the rest of the installation 3) check what will fit re minimal cost of alteration 4) fit AND commission a gas fire. Come back if you get fed a load of waffle (eg, ceramic tiles are not acceptable as a fireplace and you must have a new one etc). You also need a CO alarm if you have an open flue appliance (they are cheap on Ebay, Screwfix etc).