Little can be said of Simms's early years. At Charleston, South Carolina, on April 17, 1806, he first opened his eyes upon the world, being the second of three children, the other two of whom died in infancy. His father, for whom Gilmore was named, was a native Irishman, having emigrated to Charleston shortly after the Revolution. His mother prior to her marriage was a Miss Harriet Ann Augusta Singleton, a member of an obscure but respectable family that had moved from Virginia to Carolina sometime before the Revolution.