You’re not “adding to” but directly contradicting my statement. That is completely fine and I’m not stranger to criticism but you should probably do some more reading on the subject:

(1) Modern gas fireplaces in the US are more likely to be ventless than vented.

(2) ventless fireplaces do not require an open window when in use. That’s absurd. Imagine opening your living room window because it’s 15 degrees out and you want to warm the living room while you watch a movie on the couch with your family or because you want an ambience on Christmas Day.

It’s all based off of efficiency which results in cleaner burning and less fumes.

To add to this:

Modern gas fireplaces are typically ventless and directly dump heat into your home. That will result in you heating your living room and sweating some people out but it’s not really a heat source to heat your home (like the above comment stated).

Your traditional fireplace should have a vent leading out of the home through either a chimney or metal pipe. Turning this into a gas burning fireplace would work but all of your heat is going to vent straight out of your home. (Again, just reiterating that it would be an even less effective heat source than a ventless, gas fireplace already is.)