Theoretically ghosts were supposed to beat psychics, but Gengar was weak to psychic because he's part poison. It might have been better if another ghost type was introduced in GSC, but all we got was Misdreavus. However, they needed something since psychic moves were too strong to begin with, not being resisted by anything (besides psychic). You can see they made up a resistance in the card game (though that might be ghost vs normal). Steel also helped balance dragon a little bit, but at the time there were no powerful dragon moves. Perhaps they anticipated future additions?