Rebecca Gayheart who plays Antonia, Luke Perry's doomed Mob-princess fiancee on Beverly Hills, 90210 knew Perry's last episode would signal her end, too, but she didn't mind. After all, exec producer Aaron Spelling told her she was the first actress he'd ever hired without meeting. And after starring in the bomb Earth 2, Gayheart, 23, says, ''I wanted a short gig on a visible show. Maybe get a nice career buzz going.'' Now the former model and self-described ''Kentucky bumpkin'' has more face value than she got from her two-year stint on Loving (''a great gig, but I'd never want to do a soap again'') or her many commercials. ''People always used to think I was either their bank teller or the Noxzema girl. I always got 'Hey, you're the girl with the zits, right?' Not exactly the greatest compliment.''