Guys, Froslass and Glalie clearly don't levitate: none of them have Levitate <_<
But really it's simplest to say that they use their supernatural psychokinetic powers to float. Both Dark type and Ghost types can manifest powers similar to Psychic moves, which is what Alakazam uses to keep its head up, or how Claydol levitates.
Don't know why we're talking about Glalie anyway, it's not really a very good pokemon. Froslass can be good, though.