You should consider a direct vent system. They are available as inserts for existing fireplaces or as fireplaces (to install where there is none or to replace a fireplace). If you are unfamiliar with direct vent, do a search in this forum to find out about them.

Direct vent gas inserts are available in different sizes. They are essentially fireboxes inserted into existing fireboxes. So you can figure out if a particular system will fit your fireplace.

If you do a google or bing search of direct vent gas inserts, the addresses of websites of various manufacturers will appear. So you can do you preliminary shopping on the Internet. Be aware that direct vent systems are costly and a bit complicated to install. So you need to shop around and make sure you have a really competent installer.