They aren't really.
*G is a departure level access.on any *G airline. It's only the US carriers which only allows this on itineraries which are international. Note the careful choice of words in that sentence.

Business class generally gives you business class access at the departure airport.

Where it gets weird is when you fly on your airline where you have status and are arriving on a flight where you had departure lounge access at an airport where you can access a lounge from that airline. Then the entry requirements may be more lenient.