Herakles and Olympic gates

Are there any classical writings on the meaning behind Herakles ascending to godhood and then becoming the gatekeeper to Olympus? Who could he possibly be keeping out anyway? I could see it as meaning that he was the last mortal to attain godhood, and now serves the divine purpose of keeping mortals well.....mortal? Or at least, never attaining a level beyond that of the ancestor daimones.