Gengar are not, in fact, the ghosts of dead Clefable. They are the opposites of living ones, and comparing them to Plusle and Minun would be a far better analogy. The Clefable are a species warped by lunar rocks, never reconciled to their exile, who sneak aboard spaceships after thousands of years, and pray for a way back every night of the full moon. Gengar, on the other hand, love the Earth and evolve through events which free them of their grudges and unfinished business and allow them to move on at last. (A link cable produces a similar effect.) Unlike their gaseous predecessors, they have made peace with the fact of their death and have willed themselves back into solid forms so they can walk the planet once more.
But although they have become solid again, Gengar are not yet living, and they are far too cognizant of this reality. They are vulnerable to the Sun, but despise the Moon, and spend their days wandering in the shadows, surreptitiously planting trees and using Shadow Ball on polluters. It is not the environment they love, but what it symbolizes: some may do Articuno's bidding in icy mountains, but it is Moltres and Ho-oh which represent their hopes and dreams.
They too will pass on. They are so attached to the world that they can not bear to part from it, and possess an amazing strength of spirit. It is for this reason that the gods of the dead have allowed them to walk the Earth until that eagerly-awaited time comes and they can at last be reborn. Gengar do not seek Nirvana, for there is nothing it offers them which is better than life itself.