A gas stove is very close to 100% efficient at converting gas to heat. It is certainly not 100% efficient at putting that heat into the pot. Gas produces heat by combusting with oxygen and essentially generating hot air. The air will impart some of its heat to the pot. A ton of the heat will end up just heating the air around the stove (or being dumped outside if you’re running a hood vent).
It’s worth noting that natural gas transmission can’t be 100% efficient either. I don’t know what the loss is but it certainly takes energy to push natural gas through pipelines and there is loss in the system due to leaks.
My gas stove heats the pot, but also the cabinets above and around the stove and the entire small kitchen. Just being able to feel the air temperature go up by a couple of degrees would certainly suggest it's throwing off more waste heat than what's actually going into the pan.
Thermal electricity genetation is at best 60% efficient, and transmission losses add a couple percent.
Do you suppose that gas stove wastes a lot of heat when heating a pot that stands on it?