Oh, lawd. By the time you quit playing, Gengar simply got even more popular. By the time Platinum: Arceus came out, the deck Cursegar, using the Curse Gengar from Arceus, became very popular because it could hit-and-run and lock your opponent's Trainers at the same time. Once Undaunted came out, Gengar SF became even better, because the new Vileplume from that set locked all Trainers while it was in play, powering up Poltergeist.
In the next format, HS-on, there is only one viable Gengar deck, being Lostgar. It uses Gengar Prime and Lost World. Gengar Prime lets you look at your opponent's hand and put one Pokemon from your opponent's hand and into the Lost Zone for each Psychic Energy attached to Gengar. If your opponent has 6 Pokemon in the Lost Zone and Lost World is in play, you declare yourself the winner. This deck isn't as good as other decks, since it has no way of doing damage, and plus, there a few popular Darkness types. However, it's still a decent deck.