Terry Collins understands Dillon Gee's frustration at losing his place in the New York Mets' starting rotation. ''As I told Dillon, I don't expect him to like it, but he's got to accept it and move on,'' the Mets manager said before Tuesday night's series opener against the San Francisco Giants. Gee returned from a strained right groin that sidelined him for a month and allowed seven runs - three unearned - in four-plus innings during a 7-3 loss at San Diego last Wednesday.