“I kind of feel guilty that this is what’s happening,” laughs Grant Gustin, “Glee’s” newest hire, on the phone from L.A. He’s referring to his charmed existence over the past few years that led from his sophomore year of college to a national Broadway tour and into the arms of one of TV’s most beloved shows. Until being cast as Sebastian, the new “gay villain” appearing soon on this “Glee” season, the 21-year-old had never been on a TV or film set before. On his first ever Broadway audition in New York during a college spring break he landed Baby John in the first national touring cast of “West Side Story.” It would probably be wise for him to buy a lotto ticket or two with his current luck.