I don't think you can generalize American fireplaces. Every fireplace I've personally encountered has been traditional wood burning and absolutely heats up the room well past the time the fire is actually lit (American Northeast). I haven't actually been in a home with a gas-burning fireplace.

But more importantly - the gas fireplaces are intended to look nice with minimal effort. They are explicitly not intended to change the indoor climate much if at all. They're usually built in very new homes that have dedicated, reliable climate control systems or in cities that don't require much heating.