Looking for efficient and attractive gas heating to install into existing fire grate.

I have a couple or large fireplaces in my apartment. They are not inglenook types. Presently they just stand there, empty, which seems such a waste. So I want to install some kind of efficient and attractive gas device into the fireplaces. I would like it to employ some kind of heat exchanger, radiant, convector or fan to increase efficiency to the maximum.
Could somebody direct me to any suppliers?

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