Soon after arriving in Nauvoo, Illinois in 1844, he was called to serve as a bodyguard to Joseph Smith Jr. One special assignment involved attending a meeting of anti-Mormons who were plotting the assassination of Smith.History of the Church, 6:502-3 Belnap was at Carthage Jail on June 27, 1844, the day Smith was killed. He and Porter Rockwell were reportedly the first Latter-day Saints who had not been in Carthage at the time of the martyrdom to hear the tragic news.Belnap, Brent J. "Life Story of Gilbert Belnap" (1996) In Nauvoo he was also ordained to the Sixth Quorum of Seventy.
