A former U.S. Marine recruiter identified by authorities as a person of interest in the disappearance of a 21-year-old Winthrop man was held on unrelated gun charges yesterday in court, where relatives of the missing man voiced their anger.Frank Walter Lipka, 39, is being investigated by Boston police for the disappearance of his former roommate and Marine recruit, Joseph Brancato, 21, prosecutor Michael Lefleur said.