In theory gas boilers for heating are above 90% efficient. Not 100% because to achieve 100% what you'd have to do is keep the exhaust gases (which are hot) inside, where the people are, and unfortunately the exhaust gases are poisonous so that's a terrible idea.

To hit 90% the boiler needs to be designed to condense water vapour out of the exhaust gases, this way we'll get back the energy needed to turn water into a vapour which is a large portion of the energy embodied by the exhaust gas. And to do that the vapour needs to pass a low temperature fluid, so we use the input fluid we were about to heat with the boiler anyway, we want this fluid to be cooler than about 55°C but that means if we're using the boiler to heat a home with radiators, rather than to make fresh hot water for cleaning etc. we need our return temperature from the radiators to be less than 55°C which means we need our flow temperature to be lower (than the typical 70-80°C programmed by builders, not lower than 55°©) or else the radiators can't possibly radiate enough heat to hit that number, which means we're actually doing much of the same heating efficiency work we'd have to do to use heat pumps anyway...