Kim Zimmer, the three-time Emmy winner who has returned as larger-than-life Reva on CBS' Guiding Light, says she is not allowed ''to use the G-word.'' Ghost, that is. Reva, who ''died'' in 1990, has apparently risen from her watery grave. ''Let's say she's stuck between heaven and hell.'' A year ago, Zimmer was too unable to find prime-time work. But this season Zimmer, 40, landed a splashy recurring role on Models Inc., followed by appearances this month on Seinfeld, University Hospital, and Babylon 5. Even so, the busy Zimmer has returned to GL for this 12-week stint. ''I'd have signed on for the rest of my life,'' she says. ''I've still never found a role that challenged me like Reva. In daytime, we make s--- smell like roses. I've missed that.''